Organic Foliar Treatments for Potato Late Blight Control

A. V. Sturz¹, D. Lynch², R. Henry³, and S.W. Watts⁴

Potatoes are an important agricultural crop in the Maritime Provinces, particularly in Prince Edward Island and New Brunswick. The organic potato market is expanding, due in part to consumer concerns over environmental sustainability and food safety. However, because of restrictions on pesticide use, disease management can be a challenge in organic potato production.

Late blight (caused by Phytophthora infestans) can be a particular problem to organic potato growers. The disease affects both the foliage and the tuber, and decreases yields. When conditions are favourable the disease develops and spreads very quickly. Organic producers have relied primarily on copper products for control of this disease, but concern over copper accumulation and its toxic effects on the soil ecosystem means that a wider range of control options is needed.

Compost teas and powdered kelp products may have some potential in the control of late blight, but further information is needed on their efficacy and mode of action. Compost teas are believed to inoculate the plant surface with microorganisms that antagonize or compete with plant pathogens. They may also decrease pathogen resistance, inhibit pathogen spore germination, or reduce lesion expansion. The mode of action of powdered kelp products is also at this point largely unknown, but may be related to increased salt concentration on the plant leaf surface and/or improved plant health.

To provide more information in this area, a field and laboratory study evaluated the efficacy of commercially available compost tea and powdered kelp products against the potato late blight pathogen (Phytophthora infestans). The following treatments were included:

• JF Compost Tea (Soil-food Web, Inc.) at 69 L per ha;
• ASL Powdered Kelp Product (Acadian SeaPlants Inc.) at 300 g/ha;
• Manzate® 75 DF (V) at 1.25 kg per ha (750g/kg a.i. mancozeb);
• Well water with no added products*;
• Untreated control (i.e. no sprays of any kind).

*Since well water was used to mix and apply the treatments, this established a reference point for the effects of the pond water.

Field plots of potatoes (cv. Russet Burbank) were set out at the Eric C. Robinson Inc. Farms in Albany, Prince Edward Island. Information on the microbial population density, species make-up, and community structure was recorded both for the tank mixes and for leaf washings taken following treatment with the sprays. In addition, individual bacterial isolates from the tank mixes and leaf washings were incubated in the presence of the late blight pathogen. (Isolate selection was based on the percent frequency of occurrence of individual species in each specific community sampled.) The potential for each bacteria strain to inhibit pathogen growth (evidence of "antibiosis") was estimated in laboratory experiments using the growth rate of the fungus (in mm) as a measure of biocontrol ability.

Results from the field plot tests and bacterial incubation studies indicate that:

• The tank mixes contained bacterial isolates with significantly higher antibiosis activity against the potato late blight pathogen (P. infestans) than did the well water carrier. However, this microbial community activity was not transferred from the tank mixes to the plant leaf surface.
• Bacterial communities on the leaf surfaces after treatment bore little or no resemblance to the bacterial communities in the tank mixes. Therefore, the bacterial communities in the tank mixes either failed to become established on the leaf surface, or were washed off during application.
• Bacterial isolates recovered from the potato foliage following the foliar treatments were less effective at inhibiting late blight (P. infestans) growth than were the resident (i.e. pre-treatment) bacterial populations.
• Bacterial population densities on the leaf surface were significantly reduced following application of the commercial foliar treatments.

In a second laboratory test the relative protectant ability of the sprays was evaluated using whole leaves from potato plants that had previously been sprayed with the treatments. The leaves were inoculated with a droplet in which P. infestans zoospores were suspended and then incubated for 5 days. After incubation the number of diseased leaves, the amount of diseased leaf tissue, and the diameter (mm) of the largest lesion on each leaf was recorded.

Results from this trial indicate that:

• Neither the JF Compost Tea nor the ASL powdered kelp was effective in protecting potato leaves from phytopathogen attack.
• Manzate® 75 DF was the best treatment for controlling late blight infection and disease development in this trial. However, results from the bacterial incubation trial showed that Manzate® 75 DF also reduced the number of bacterial strains with biocontrol activity against the late blight pathogen (P. infestans) in comparison to the pre-treatment bacterial populations.
  

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Further information on organic control strategies for late blight can be found at: Kuepper, G. and P. Sullivan. 2001. Organic Alternatives for Late Blight Control in Potatoes. Appropriate Technology Transfer for Rural Areas (ATTRA) Pest Management Technical Note.


Author Locations and Affiliations
(1) Department of Plant Health Research & Diagnostics, PEI Dept. of Agriculture, Fisheries, Aquaculture and Forestry, P.O. Box 1600, Charlottetown, PE C1A 7N3, E-mail: avsturz@gov.pei.ca
(2) Organic Agriculture Centre of Canada, Nova Scotia Agricultural College
(3) Roger Henry and Associates, Agro - Environmental Technologies, Stanley Bridge, PEI
(4) S.W. Watts, Eric C. Robinson Inc./Ingleside Farms, Albany, PE


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