Pest Management in Potato

Potato scab and Colorado Potato Beetle (CPB) are a serious problems for Saskatchewan potato growers. Scab levels above commercial tolerance levels result in grade outs and reduced crop quality, and defoliation of potatoes and other Solanaceous crops by CPB may cause yield losses, with the extent of damage depending on insect populations and the vigor of the crop

Trials conducted by the Vegetable Program have evaluated varying agronomic practices and biocontrol options for the management of scab and CPB.

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Disease Management Articles

Delaying Harvest Increases Losses to Common Scab in Potatoes PDF
In 1998 and 1999 we specifically evaluated how yields and grade-out to scab changed as the interval between top kill and harvest increased.

Integrated Crop Management Practices for the Control of Common Scab of Potatoes PDF
Potato scab is a serious problem in Saskatchewan as scab levels above commercial tolerance levels results in grade outs and reduced crop quality. Over four growing seasons this project has evaluated agronomic practices which could be used to control potato scab in Saskatchewan.
Project 1 Soil pH Managment
Project 2 Delaying Harvest Increases Losses to Common Scab in Potatoes
Project 3 Canopy Management to Control Common Scab
Project 4 Time of Planting
Project 5 Sulphur (S) as a Component on Nitrogen (N) Fertilization

Manuring as an Option for Management of Potato Scab PDF
Traditionally growers have been cautioned to avoid manuring potato fields for fear of worsening problems with scab. However, recent studies show that application of fresh manure prior to planting may reduce common scab levels in infested fields.This trial attempted to evaluate the potential to control scab utilizing readily available manure sources.

Insect Management Articles

Evaluation of Biocontrol Agents for Colorado Potato Beetles PDF
Defoliation of potatoes and other Solanaceous crops by Colorado Potato Beetles (CPB) may cause yield losses, with the extent of damage depending on insect populations and the vigor of the crop. This trial evaluated the biocontrol agents Bacillus thuringiensis and Beaveria bassiana for their ability to control CPB populations in an irrigated potato crop in Saskatchewan.

Powerpoint Presentations:

For more presentations visit the Presentations page

Disease Management Products for Potato Storage (PPT - 1.4 MB)
This presentation covers potato storage issues and potential disease/rot pathogens, and discusses a range of products available for the management of potato storage.

Managing Potatoes Under High Temperature Conditions (SHOW - 3.2 MB)
This presentation discusses how unusually hot growing seasons affect potato crop growth, tuber quality and storability, and disease and insect problems. It suggests steps growers can take to minimize damage to potatoes.

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Delaying Harvest Increases Losses to Common Scab in Potatoes

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In commercial potato production, it is not uncommon for harvest to be delayed for several weeks after the tops have naturally senesced or been chemically desiccated. Delaying harvest is known to increase the incidence of Rhizoctonia (Black Scurf) on the tubers. Trials conducted by the University of Saskatchewan from 1995-1997, indicated that the incidence and severity of common scab increased the longer the crop was in the ground (ie; early planting combined with late harvest). In these trials, yields also increased the longer the crop was in the ground, which tended to offset the increased gradeout to excessive scab. In 1998 and 1999 we specifically evaluated how yields and grade-out to scab changed as the interval between top kill and harvest increased.

The trials were conducted on land heavily infected by common scab. Norland and Shepody were used as the test crops. The crops were planted in mid-May and grown using standard management practices for irrigated potatoes. The fields were top-killed in the first week of September using a full rate of diquat (1.2 l/a Regalone in 120 l water/a) applied by a ground rig sprayer. To obtain complete top kill of cv. Shepody, diquat was applied a second time, one week later (0.75 l/a). Sections of the plot were harvested; a) just prior to, b) two weeks after (3rd week of September) and c) four weeks after top killing (2nd week of October). Yields were determined, then the crop was evaluated for incidence and severity of tuber infection by common scab. Tubers were considered marketable if less than 10% of their surface was damaged by scab.

Results

Losses to scab in 1998 were higher than in 1999 but otherwise, the yield and scab responses to harvest intervals were similar in the two test years. Yields were not strongly influenced by the duration of time between the initial top killing treatment and the harvest (Table 1). This was expected for cv. Norland as the tops died very quickly, however, for cv. Shepody some yield increase had been expected due to the delay in achieving full vine kill. As expected, the incidence and severity of scab damage was more severe for Shepody than Norland (Table 1). For both cultivars, the incidence and severity of scab damage to the surface of the tubers increased with the duration of delay between top killing and harvest. The increase in grade out to scab was equivalent to or greater than any increase in yields obtained by delaying harvest. Consequently, marketable yields often declined as the duration of time the crop was left in the ground after top-killing increased.

The results clearly demonstrate the importance of harvesting as soon as possible after top killing as a means of minimizing losses to common scab. In situations where the risk of losses to scab is high (infested soils and sensitive cultivars) growers should consider top kill methods that minimize the time between treatment and harvest. Mechanical flailing can be used in conjunction with chemical desiccants to accelerate vine removal. Alternatively, growers may consider staggering top kill to match harvest capabilities, thereby minimizing the period the crop is in the ground after top-killing.

Table 1. Influence of time of harvest on yields and incidence of scab on Norland and Shepody potatoes in 1998 and 1999

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Integrated Crop Management Practices for the Control of Common Scab of Potatoes.

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Potato scab is a serious problem in Saskatchewan as scab levels above commercial tolerance levels results in grade outs and reduced crop quality. The pathogen causing scab is widely distributed in Saskatchewan and cannot be effectively controlled by crop rotation or chemical control methods. For the last four cropping seasons, this project has evaluated agronomic practices which could be used to control potato scab in Saskatchewan.

• Project 1 Soil pH Managment
• Project 2 Delaying Harvest Increases Losses to Common Scab in Potatoes
• Project 3 Canopy Management to Control Common Scab
• Project 4 Time of Planting
• Project 5 Sulphur (S) as a Component on Nitrogen (N) Fertilization

Project 1. Soil pH Managment

The common scab organism (Streptomyces scabies) prefers soil pH from 5.5 - 8.0, which corresponds to the pH of most Saskatchewan soils. This trial (1995-1998) examined whether scab could be controlled by shifting the soil pH, without adversely affecting the crop. Soil pH in test plots was altered from the original level (pH 8.0) by applying varying amounts of CaCO3 or S prior to planting.

The moderately scab resistant variety Norland was used as the test variety in most trials. In 1998, the more scab sensitive cv. Shepody was also examined. Each test plot consisted of four, 8 m long rows arranged in a randomized complete block design with four replicates. Standard production and irrigation practices were employed. The crop was harvested and graded for yields and 100 tubers were evaluated for scab damage. In 1998, Colorado Potato Beetles caused extensive defoliation of cv. Shepody late in the season, resulting in poor yields for this variety.

The pH treatments tested in 1998 ranged from 7.5 to 9.5. As found in previous years, yields of marketable size tubers were unaffected by soil pH (Table 1). As expected, the incidence and severity of scab on cv. Shepody was much more severe than for cv. Norland. Soil pH had little effect on grade out to scab and yields after grading for cv. Norland (Table 1). For cv. Shepody, raising the pH to 9.0 or above reduced grade out to scab relative to the original soil pH (8.0). Yields after grade out to scab were also highest at pH levels of 9 and above (Table 1).

Table 1. Influence of soil pH on yields, grade out to scab and yields after grade out for Norland and Shepody potatoes (Saskatoon - 1998).

Table 2. Influence of soil pH on grade out to excessive scab for Norland potatoes (Saskatoon 1995-1998).

Conclusion

Over a range from 6.5 to 9.5, soil pH has little impact on yields of potatoes under Saskatchewan growing conditions. The potential to control scab by altering the soil pH appears to have promise, although the results vary from year to year. In three of four test years (1995, 1996 and 1998) adjusting the soil pH upwards by liming reduced the incidence and severity of scab on the crop, relative to the original soil pH (7.5-8.0) (Table 2). By contrast, in 1997 reducing the soil pH to 7.5 reduced scab levels relative to the control (pH 8.5), while shifting the pH to higher levels worsened the scab situation. Growers considering pH modification as a means for scab control are cautioned that the availability of some soil nutrients begins to decline at high pH levels and that treatments applied to modify soil pH may persist for several years.

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Project 2. Delaying Harvest Increases Losses to Common Scab in Potatoes

In commercial potato production it is not uncommon for harvest to be delayed for several weeks after the tops have naturally senesced or been chemically desiccated. Delaying harvest is known to increase the incidence of Rhizoctonia (Black Scurf) on the tubers. Trials conducted by the University of Saskatchewan from 1995-1997, indicated that the incidence and severity of common scab increased the longer the crop was in the ground (ie; early planting combined with late harvest). In these trials, yields also increased the longer the crop was in the ground, which tended to offset the increased gradeout to excessive scab. Trials in 1998 specifically evaluated how yields and grade out to scab changed as the interval between top kill and harvest increased.

The trial was conducted on land heavily infected by common scab. Norland and Shepody were used as the test crops. The trial was planted in mid-May and was grown using standard management practices for irrigated potatoes. The field was top-killed on September 5 using a full rate of diquat (1.2 l/a Regalone in 120 l water/a) applied by a ground rig sprayer. To obtain complete top kill of cv. Shepody, diquat was applied a second time, one week later (0.75 l/a). Sections of the plot were harvested; a) just prior to, b) two weeks after (September 19) and c) four weeks after top killing (October 9). Yields were determined, then the crop was evaluated for incidence and severity of tuber infection by common scab. Tubers were considered marketable if less than 10% of their surface was damaged by scab.

Results

Yields were not influenced by the duration of time between the initial top killing treatment and the harvest (Table 3). This was expected for cv. Norland as the tops died very quickly, however, for cv. Shepody some yield increase had been expected due to the delay in achieving full vine kill. As expected, the incidence and severity of scab damage was more severe for Shepody than Norland (Table 3). For both cultivars, the incidence and severity of scab damage to the surface of the tubers increased with the duration of delay between top killing and harvest. For example, for cv. Norland, if the crop was harvested on September 5, more than half of the tubers had only a trace of scab, but by the October 9 harvest only 13% of the tubers were still completely scab free. For cv. Norland, the percentage of the crop meeting marketing standards for freedom from scab declined from 83% on September 5 to 52% by the October 9 harvest. Factoring in the change in total yields over the three harvest dates, harvesting on September 5 resulted in 26% higher marketable yields versus the October 9 harvest. For cv. Shepody, the percentage of the crop meeting market standards for scab declined from 18% on September 5 to only 2% by October 9. This resulted in a 80% reduction in marketable yields.

Table 3. Influence of time of harvest on yields and incidence of scab on Norland and Shepody potatoes.

* <10% of tuber surface covered by scab lesions.
** Harvest 1 = at top kill, Harvest 2 = two weeks after top kill, and Harvest 3 = four weeks after top kill

Conclusion

The results from this trial clearly demonstrate the importance of harvesting as soon as possible after top killing a means of minimizing losses to common scab. In situations where the risk of losses to scab is high (infested soils and sensitive cultivars) growers should consider top kill methods that minimize the time between treatment and harvest. Mechanical flailing can be used in conjunction with chemical desiccants to accelerate vine removal. Alternatively, growers may consider staggering top kill to match harvest capabilities, thereby minimizing the period the crop is in the ground after top-killing.

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Project 3. Canopy Management to Control Common Scab

The incidence and severity of scab in potatoes is increased by drought stress at the time of tuber set. Although growers strive to minimize this stress by careful moisture management, problems with scab are still common. Treatments which limit crop moisture use at the critical stage of development may be useful in preventing scab. In trials conducted from 1995-1997, foliar application of the growth inhibitor Daminozide had reduced grade out to scab but also had reduced total yields. Similarly, partial defoliation of the plants at tuber set had reduced losses to scab but also reduced yields.

In 1995, the growth regulating treatments tested had no effect on pre-grading yields. The growth retardant Daminozide had a strong inhibitory effect on scab, while the other treatments were no better than the control. The final marketable yields for the Daminozie treated crop were significantly greater than all the other treatments.

In a 1996 trial, partial defoliation of foliar application of growth regulators like Daminozide and ABA at the time of tuber set reduced the incidence of scab in heavily infested soils. Treatment effects likely stemmed from a reduction in water use triggered by the treatments; the incidence of scab is known to be inversely related to soil moisture availability at tuber set. While these treatments provided a degree of scab control they did so at the expense of overall yields.

The 1997 trial used Norland and Shepody potatoes at the Saskatoon test site. The treatments were; 1) control, 2) Daminozide (5g/L) applied to run off at stolon initiation. Daminozide is a growth retardant, which shifts root/shoot ratios in favour of the roots. 3) Partial defoliation at tuber set - the top 40% of the canopy was removed using a "weed-eater" to slow transpiration, thereby maintaining a favourable moisture balance for the developing tubers. 4) Abscisic acid (ABA) at tuber set (15% s-ABA applied at 55 ml/a). ABA slows growth and reduced water use by temporarily closing the stomata. 5) Antitranspirant (Vapor-Lok) - temporarily close the leaf stomata, thereby reducing moisture loss.

In 1998 we again examined the impact of a) foliar applied Daminozide (5g/l) and 2) partial defoliation on yields and scab levels in Norland and Shepody potatoes.

Trials were conducted in Saskatoon, under irrigation on land heavily infected with common scab. Treatments were applied in early July, at which time the crop was just beginning to set tubers. The Daminozide was applied in 100 l/ha of water using a CO2 powered backpack sprayer. A “weed eater” was used to remove about 75% of the top growth in the defoliation treatment. The crop was managed, harvested and evaluated for scab in the usual manner.

1997 Results

The two cultivars responded similarly to the treatments (Table 4.1). None of the treatments produced any noticeable change in crop appearance. The defoliated plots recovered very quickly and were indistinguishable from the rest of the trial at harvest. Total yields for Norland were not affected by any of the treatments. In Shepody, the ABA and defoliation treatments influenced the gradeout to excessive scab which in in contrast to results from previous years. Grade out to scab for all treatments in the 1997 trial was substantially lower than in previous years ie; the least effective treatment in 1997 had a higher percentage of marketable tubers than the most effective treatment in previous years. It appears that conditions in 1997 at tuber set were not conducive to scab formation. After grade out to scab, there were no significant differences in yields for any of the treatments.

Table 4.1. 1997 results - Influence of canopy control methods on yields and grade-out to common scab.

* Values within the same columns followed by the same letter are not significantly different.

1998 Results

Defoliation significantly reduced yields for both cultivars, while the Daminozide had no effect on yields (Table 4.2). Grade out to scab was much higher in cv. Shepody than in cv. Norland. In cv. Norland, neither treatment had a significant impact on grade out to scab (Table 4.2). After grade out to scab, yields for the Daminozide treatment were significantly higher than the control while defoliation reduced yields relative to the control. The treatments had a more pronounced effect on scab in the more sensitive cv. Shepody. Defoliation reduced grade out to scab by 42% relative to the control, while the Daminozide reduced grade out by 11% (non-significant). Yields after grade out to scab for the Daminozide treatments were significantly better than the controls (Table 4.2). While defoliation helped control scab, it also reduced yields, resulting in no improvement in marketable yields relative to the controls for cv. Shepody.

Table 4.2. 1998 results - Influence of canopy control methods on yields and grade-out to common scab.

Conclusion

Treatments which slow crop water use at the critical stage of tuber initiation have the potential to reduced losses to common scab. The challenge is to identify treatments which consistently control scab without excessively compromising yield potential. In this trial, the growth retardant Daminozide provided the most consistent positive effects by providing a degree of scab control without excessive loss in yield potential. The defoliation treatment caused more extensive reductions in yields than in previous years - this likely reflects the fact that 75% of the foliage was removed as opposed to 40-50% in previous years.

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Project 4. Time of Planting

Planting time can influence soil temperatures and evaporation rates during tuber set. If early planting results in tuber set during the relatively cool weather of early summer there should be relatively fewer problems with scab that if tubers were set during more drought prone weather. This trial examined the relationship between time of planting and the incidence of common scab in potatoes.

Trials were conducted from 1995 to 1997 using Norland and Shepody potatoes. The early crop was seeded in the 2nd week of May while the late crop was seeded 4 weeks later. Standard management practices were employed. Each plot consisted of 3, 8m long rows, arranged in a randomized complete block design with four replicates. The trial was laid out to allow for three harvest dates (early August, late August, and mid-September). The crop was graded and evaluated for scab as previously described.

The yield and scab level responses for the Norland and Shepody potatoes were similar in years and consequently only the Norland data is presented.

1995 Trial

Delaying planting by 4 weeks had relatively little effect on yields, unless the crop was also harvested early (Table 5). Grade-out to scab was high in this trial. The combination of early planting and delaying the harvest resulted in a substantial increase in grade-out to excessive scab. As a consequence, planting late and harvesting early produced the highest yields of scab free tubers in 1995.

1996 Trial

Delaying planting by four weeks resulted in a substantial reduction in yields in 1996 (Table 5). In the early planted crop, delaying the harvest until September was not necessary to produce maximum yields. The late planted crop benefited from delaying the harvest as long as possible. Severity of grade out to scab increased as harvest was delayed, although the impact of the delay on scab severity was more pronounced in the late planted crop. The early planted crop, harvested in early September produced the highest yields after grade out to scab.

1997 Trial

Yield and scab responses to the planting and harvesting variables in 1997 were very similar to the 1996 trial (Table 5). Delaying planting by 4 weeks had a negative effect on yields, particularly if the crop was harvested early. Severity of grade out to scab was not influenced by the planting date, but increased as harvest was delayed. The early planted crop, harvested in early September produced the highest yields after grade out to scab.

Table 5. Influence of planting and harvest time on yields, grade out to excessive scab, and marketable yields of Norland potatoes (Saskatoon 1995-1997).

Conclusions

Delaying planting slowed crop development, resulting in some loss in yield potential in most cases. Delaying the harvest rarely was beneficial in the early maturing Norlands. Grade out to scab increased with the duration of time the crop was in the field. This is to be expected as the scab lesions increase in size over time, and once the lesions cover more than 10% of the surface of the tuber it is no longer marketable. Extending the duration of the growing season by delaying the harvest resulted in more scab problems than did early planting. This suggests that environmental conditions in the fall may be more conducive to the development of scab than earlier in the season. The optimal time to harvest represented a balance between the anticipated increase in yields obtained by delaying the harvest relative to the associated increase in grade out to excessive scab. For Norland potatoes which senesce relatively early, the balance favoured minimizing the period the crop was in the field.

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Project 5. Sulphur (S) as a Component on Nitrogen (N) Fertilization

Application of S is commonly recommended for scab control. Application of elemental S is limited by its slow availabiliy and if the addition of S decreases the soil pH, problems with scab may actually increase. This trial evaluated the impact of S applied as a component of the N fertility program on yields and scab incidence.

The trial was conducted in Saskatoon using Norland potatoes. The treatments were;

• 1) NH4NO3 (preplant) + NH4NO3 (postplant)
• 2) NH4NO3 (preplant) + NH4 SO4 (postplant)
• 3) NH4 SO4 (preplant) + NH4 SO4 (postplant)
• 4) NH4 SO4 (preplant) + NH4NO3 (postplant)

Both the preplant and postplant N application provided 50 #/a actual N. Each test plot consisted on four, 8 m long rows arranged in a randomized complete block design with four replicates. Aside from the fertilizer treatments, standard production and management practices were employed. The crop was harvested and evaluated for scab incidence as previously described.

1995 Results

Yields were very high in all treatments (average 50 t/ha) and there were no differences between marketable yields for the various treatments prior to scab grade out i.e. marketable size (Table 6). The two treatments which featured both the NH4SO4 and NH4NO3 had more tubers free of any scab than the other fertilizer strategies. Marketable yields (Marketable size - grade out to scab) were also significantly higher for the two mixed fertilizer treatments.

Table 6. Influence of fertilizer formulation and yields and scab infection for Norland potatoes (Saskatoon, 1995).

* Values within columns followed by the same letter are not significantly different (P = 0.05)
** Marketable yield = marketable size * % crop with no or low scab incidence

1996 Results

Again, there were no significant differences in the total yields for the various treatments (Table 7). The treatments which included NH4SO4 had more tubers free of scab than when only NH4NO3 was applied (Table 7). Marketable yields were highest in the treatments which featured NH4SO4 at planting. As in the 1995 trial, the regime based strictly on NH4NO3 produced the lowest yields after grade out to scab.

Table 7. Influence of fertilizer formulation and yields and scab infection for Norland potatoes (Saskatoon, 1995).

* Values within columns followed by the same letter are not significantly different (P = 0.05)
** Marketable yield = marketable size * % crop with no or low scab incidence

Summary

Addition of NH4SO4 to the fertilizer progrom provided a degree of scab control without a significant sacrifice in yields. S application are thought to be effective at controlling scab, yet the treatment featuring two applications of SO4 was no more effective than the single application. Applying the SO4 at planting or in the top dressing made no difference in 1995, but in 1996 the preplant application was most effective. This difference may reflect difference in timing of the top dressing relative to the sensitive stage of tuber development. The question of optimum timing of SO4 fertilizers mertits further investigation.

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Manuring as an Option for Management of Potato Scab

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Traditionally growers have been cautioned to avoid manuring potato fields for fear of worsening problems with scab. However, Conn and Lazarovits (1998) found that application of fresh poultry or swine manure just prior to planting actually reduced levels of common scab in a severely infested field. Researchers in Japan have isolated micro-organisms from manure which actively combat scab. This trial attempted to evaluate the potential to control scab utilizing readily available manure sources.

The trial was conducted on the Department of Plant Sciences Potato Research station in Saskatoon. This area has a long history of potato production and is severely infested by both common (Streptomyces scabies) and powdery (Spongospora subterranea) scab. The manure source tested was turkey manure mixed with some pen straw. The manure was collected over a 3 month period. Just prior to seeding the crop, the manure was applied at 0, 10 or 20 t/acre and then incorporated to 15 cm utilizing a rotovator. No additional fertilizers were applied to the plot area as soil tests indicated the presence of adequate levels of all macronutrients.

Norland which is partially resistant to scab and Shepody which is more scab sensitive were planted at 23 cm in-row spacing with 1 m between rows. Half of each plot was irrigated while the remainder relied on rainfall. Standard production practices were employed for the duration of the growing season. The crop was top-killed in mid-September and harvested in early October. The crop was graded and then evaluated for the incidence and severity of scab. Tubers with more than 10% of their surface covered with scab lesions were graded out after which marketable yields were re-calculated. .

Results

Rainfall was extremely limited in 2001, resulting in exceptionally poor yields under dryland conditions. Yields under irrigation were well above normal in the control treatments (Table 1). Under both irrigated and dryland conditions, application of manure significantly reduced yields. Plants in the manured areas emerged slowly and appeared stunted throughout the growing season. Roots in the manured areas were short and abnormally thickened. These symptoms are indicative of the salt and ammonia toxicity stresses commonly observed following application of excessive volumes of fresh manure. Application of 10 t/acre of manure had the same impact on yields as application at twice that rate.

Grade out to scab was much higher in the irrigated regime than under dryland. This suggests that powdery scab was the dominant scab organism at the site - powdery scab thrives under moist conditions, while common scab prefers warm drier conditions. Under both dryland and irrigates conditions application of manure reduced the proportion of the crop graded out due to excessive scab. This effect was not related to the amount of manure applied. In both cultivars, manuring increased the yields after grade out to scab.

Table 1. Influence of manure on yield and grade out to scab for two cultivars of potato growing under irrigated and dryland conditions.

The results support the concept of utilizing manure to suppress scab - however, the responses were variable and therefore must be interpreted with caution. The degree of yield suppression observed following application of the manure greatly exceeded our expectation. Problems with excessive salts and ammonium toxicity would be even greater if fresh manure was used - yet fresh manure is recommended by Conn and Lazarovits. The fact that both yield loss and suppression of the scab appeared independent of the amount of manure applied suggests that lower rates should be examined. The presence of both common and powdery scab on the test site complicates management of the problem - but the test it is noteworthy that the manure appeared to suppress both types of scab.

Evaluation of Biocontrol Agents for Colorado Potato Beetles

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Defoliation of potatoes and other Solanaceous crops by Colorado Potato Beetles (CPB) may cause yield losses, with the extent of damage depending on insect populations and the vigor of the crop. Consistent chemical control of CPB is complicated by their rapid development of resistance to a range of pesticide chemistries. Application of biocontrol agents may represent an alternative to standard chemical control. This trial evaluated the biocontrol agents Bacillus thuringiensis and Beaveria bassiana for their ability to control CPB populations in an irrigated potato crop in Saskatchewan.

The trials were conducted on the Plant Sciences Department Potato Research plots in 1999 and 2000. Russet Burbank (1999) or Norland (2000) potatoes were grown using standard irrigated production practices. The long history of potato production in the plot area has provided ample opportunity for establishment of a CPB population. In previous years, CPB numbers were sufficient to cause some yield loss. To increase the CPB populations in the test area, adult beetles captured in other areas were released into the plot in the first week of July. The final population of adults (ca 1/plant) approximates the economic damage threshold.

The treatments were ;

• Control - no CPB control methods applied.
• Chemical - Decis (deltamethrin) @ 150 ml/ha
• Beaveria bassiana - Mycotrol® (Mycotech Corp.) @ 2.3 1/ha
• Bacillus thuringiensis - Safers® BTKtm @ 3.3 l/ha in 1999 and B.t. tenebriones in 2000

The sprays were applied to the foliage of the crop beginning in late June 20 with the emergence of the first larva and repeated every two weeks until early August. Counts of CPB larvae and adults were done weekly. The crop was harvested in late September.

Results

From mid June through early July only overwintering adults were present in the test plots. Significant populations of larva became apparent in mid-July. Larvae numbers reached a peak within two weeks and then gradually declined over the remainder of the sampling period (Figures 1 and 2). The non-treated control treatment supported the highest numbers of larva, with the Mycotrol and Bt treatments providing about 50% control. Foliar applied Decis provided almost complete control of the CPB larvae.

By early August adult beetles of the year were beginning to appear in the test plots (Figures 3 and 4). Although these beetles cause little damage to the crop, their numbers are important as an indicator of infestation levels in the next cropping season. The numbers of adult CPB in the various treatments reflected the effectiveness of the treatments at controlling the larval stage. The control treatments supported the most adult beetles followed by the two biocontrol treatments.

Yields in both years were fairly typical of irrigated potatoes in Saskatchewan. None of the CPB control measures significantly improved yields over the non-treated controls. Typically, yield losses in potatoes due to insect damage or mechanical defoliation are not apparent until there is about 25% defoliation. In this trial, insect populations in the control treatment were insufficient to produce this level of defoliation.

Conclusion

Application of the Bb (mycotrol) and BT provided some degree of CPB control, although the the degree of control was greatly inferior to chemical control. The limited control provided by the biocontrol agents may reflect;

• a) unfavorable conditions for development of the Bb infection or survival of the BT
• b) insufficient dosage and /or insufficient time period for development of Bb populations

High humidity levels are critical to the establishment of the Bb infection - the dry conditions typical of Saskatchewan summers may not be favorable for this biocontrol agent. CPB mortality from Bb is not rapid, but it adds up in later life stages. Bb therefore has a greater impact in areas where CPB has several generations each season.

The CryIII protein of Bt has been shown to provide effective control of CPB under controlled conditions. However, its efficacy is highly dependant on the beetles consuming a fatal dosage of the bacteria before the bacteria are killed by exposure to UV light. This necessitates multiple applications (every 2-4 days) of this product.

Potatoes are relatively tolerant of defoliation, with significant yields losses only occurring once 25% of the leaf area has been lost. Although efforts were made to increase Colorado Potato Beetle populations in this trial, the numbers were still insufficient to cause significant yield losses. The cv Russet Burbank used in 1999 is a very vigorous variety which produces abundant foliage, particularly when grown under intensively managed conditions. The cv. Norland used in the 2000 trial produces a much smaller plant canopy, but it is highly tolerant of stress. This again may make this cultivar fairly tolerant of CPB damage. Higher populations coupled with reduced crop vigor due to poor genetics or inappropriate crop management would increase the potential for significant yield losses due to CPB damage.

Related Papers

Thomson, J., and D. Waterer. 1999. Biotyping of potato diseases in Saskatchewan. ADF Report No. 96000179 Final Report

Waterer, D., J. Thomson, R. Spencer. 2003. Ozone as an improved method to control disease in stored potatoes. ADF Project No. 98000285 Final Report