Screening trial for solutions to Verticillium wilt in potatoes

Project Code SCR07-003

Project Lead

Steve Howatt - Atlantic AgriTech Incorporated

Objective

To evaluate the potential of a number of reduced-risk management practices and products to control verticillium wilt of potato and to provide recommendations for candidate reduced-risk pesticides to be pursued for registration for this use in Canada

Summary of Results

In recent years, early dying complex (EDC) has become a serious problem in potato production across Canada. This disease is caused by Verticillium fungi (Verticillium albo-atrum and Verticillium dahliae).

The Verticillium fungus can overwinter in crop residue, soil and tubers and can be spread through contaminated equipment, infected soil or by wind. Infected seed potatoes cannot be visually identified and therefore cannot be graded out of a seed lot.

The objective of this project is therefore to evaluate the efficacy of certain reduced-risk control methods in reducing or eliminating pressure due to EDC.

The first part of the project involved evaluating the effects of various green manures on Verticillium levels in soil and potato plants.

The trial was conducted in New Glasgow, Prince Edward Island. The plots were inoculated with the two species of Verticillium (albo-atrum and dahliae) in early spring, before any of the treatments were applied. The various green manures (11 in total) were seeded at the end of May 2007 in 2 abbr title="meters">m by 3 abbr title="meters">m plots. At the end of June, all the plots were rototilled to incorporate the green manure crops into the soil. The green manures used were: barley, red clover, alfalfa, rye grass, canola, mustard greens, oilseed radish, broccoli, soybeans, pearl millet and sorghum-sudan grass.

The experimental design was a randomized complete block design with four replications. The potato seed (Superior variety) was planted on June 29. On July 20, each plant was inoculated with a spore suspension containing both V. albo-atrum and V. dahliae.

In-season data collection consisted of visual observations of plant vigour and maturity. Early in the season and again at the end of the season, population levels of the two Verticillium species in the soil were determined by polymerase chain reaction (PCR). One stem and one tuber sample per potato plant were also analyzed by PCR at the end of the season. The potatoes were harvested on October 25 and data on tuber counts and yields were obtained.

The PCR analyses indicated that V. dahliae levels in the soil were very high both at the start of the season and at the end of the season for all the green manure treatments. This result shows that none of the green manure treatments resulted in a significant reduction in V. dahliae levels in the soil. However, it should be noted that the potatoes grown following the broccoli plowdown had significantly lower levels of V. dahliae compared to potatoes grown following barley or alfalfa. In terms of tuber infection, no significant differences were observed between the treatments.

Results from the PCR analyses of soils indicated low initial levels of V. albo-atrum in the soil. However, the artificial inoculation in the spring increased this level, which remained unchanged until the end of the season. Therefore, none of the green manure treatments was successful in substantially reducing V. albo-atrum levels in the soil. In terms of stem infection by V. albo-atrum, Sudan grass resulted in lower levels of infection than the non-weeded control. Potatoes grown in the barley plowdown had the highest levels of tuber infection.

The second part of the project involved evaluating the efficacy of a number of reduced-risk EDC control products.

Eleven reduced-risk pesticides were evaluated. Some products were applied as seed treatments and others as in-furrow treatments. The plots were inoculated with the two Verticillium species (albo-atrum and dahliae) in early spring, before any other treatment was applied. The experimental design was a randomized complete block design with four replications. The potato seed (Superior variety) was planted on June 29. On July 20, each potato plant was inoculated with a spore suspension containing both V. albo-atrum and V. dahliae.

In-season data collection consisted of visual observations of plant vigour and maturity. Early in the season and again at the end of the season, population levels of the two Verticillium species in the soil were determined by polymerase chain reaction (PCR). One stem and one tuber sample per potato plant were also analyzed by PCR at the end of the season. The potatoes were harvested on October 25 and data on tuber counts and yields were obtained.

The PCR results showed no significant differences between the various treatments in terms of the quantity of V. albo-atrum present in the potato stems. However, a few differences were observed between the treatments in terms of the quantity of V. dahliae present in the potato stems.

In terms of the levels of V. dahliae in the tubers, reduced levels of infection were observed in tubers of two treatments, namely Reason and Captan. In terms of the level of V. dahliae and albo-atrum in the soil, no significant differences were obtained between the various treatments.

For the two components of this experiment, correlations could not be established between the visual observations (plant vigour), the PCR results and potato yields. This is related to the fact that the weather conditions were not always favourable (rain and wind) and that the potatoes were planted later than normal due to various constraints. However, certain interesting effects were observed. This experiment will have to be repeated over several growing seasons in order to validate the results obtained during this initial trial.

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