The Community Composition and Antibiosis Ability of Potato Phylloplane Bacteria Against Potato Late Blight Following Foliar Treatments with Either JF Compost Tea, ASL Powdered Kelp or Manzate®75DF Foliar Treatments

By A.V. Sturz^1*, D. Lynch² and S.W. Watts³

A field and laboratory study was established to compare the efficacy of the commercially available JF Compost Tea, ASL powdered kelp product and Manzate® 75 DF against the potato late blight pathogen Phytophthora infestans.

Field plots of potatoes, cv. Russet Burbank, were set out, as part of a larger experiment, in a randomized complete block design with four replicate plots and three treatments, at the Eric C. Robinson Inc. Farms, Albany , Prince Edward Island, Canada.

Applications of the Jolly Farmer (JF) compost tea, made under licence through Soil-food Web, Inc. ™, were applied at a rate of 69 L per ha; an experimental powdered kelp product (Acadian SeaPlants Inc.) (ASL) was applied at a rate of 300 g/ha, and the protectant fungicide Manzate® 75 DF (Griffin, Canada) applied at the recommended rate of 1.25 kg per ha (750g/kg a.i. mancozeb). A local source of pond water was used to mix and apply the treatments.

The population density, community structure, species complement and antibiosis ability in each foliar treatment were compared to the resident phylloplane communities present on potato foliage (cv. Russet Burbank) pre- and post-foliar treatment application. The in vitro tests consisted of a series of Petri plate challenges on rye agar, whereby individual bacterial isolates were incubated for 5 d at 25 ^oC in the presence of the test pathogen and examined for inhibition of pathogen growth (evidence of antibiosis), scored by measuring the growth of the fungus (in mm).

In an in vivo test the relative protectant ability of foliar treatments was examined using whole leaves from potato plants excised and inoculated with a 20 uL droplet in which P. infestans zoospores were suspended at a concentration of 1,000 zoospores per ml then floated on sterile distilled water in Petri plates. After 5 days incubation the percent leaf tissue diseased was estimated, the number of leaves diseased counted and the diameter (mm) of the largest lesion on each leaf measured.

From the in vivo and in vitro tests, tank-mixes of foliar treatments were found to contain bacterial isolates with significantly (P=0.05) higher antibiosis ability against P. infestans than the "pond water" carrier (Table 1). However, this activity was not transferred to the phyllobacterial community following application of the foliar treatments (Table 1). Bacterial population densities on the leaf surface were significantly (P=0.05) reduced following foliar treatments with Manzate®75 DF, ASL powdered kelp and JF Compost Tea, as compared to ‘Pre-treatment' foliar populations. Bacterial isolates recovered from potato foliage following treatments were significantly less effective at inhibiting P. infestans growth in in vitro tests compared to the resident ‘Pre-treatment' foliar populations (Table 1).

Bacterial communities on post-treatment leaf surfaces bore little or no resemblance to the bacterial communities in the spray tank mixes prior to application, and failed either to become established on leaf surfaces or may simply have been washed-off during foliar application.

Manzate®75 DF was the best treatment for controlling P. infestans infection and disease development in vivo (Table 2). Both JF Compost Tea and ASL powdered kelp were similar in their inability to protect potato leaves from phytopathogen attack. However, the phyllobacterial populations established following Manzate foliar applications proved to have significantly (P=0.05) fewer antibiotic producing biocontrol strains (in vitro) than those naturally occurring leaf populations prior to foliar treatment (Table 1).

Table 1. Numbers of bacterial colony forming units (CFU) and overall antibiosis ability of bacterial communities recovered from pond water (diluent), tank mixtures and potato foliage.

a Source of water for all the tank-mixtures and applications.
b Manzate® 75 DF.
c Numbers in rows followed by the same letter are not significant different using a protected Least Significant Difference test (P= 0.05).
d No data collected.
e The average level of growth inhibition of Phytophthora infestans (evidence of antibiosis) was calculated by measuring the ability of 800 individual bacterial isolates (200 isolates per treatment) to inhibit the growth of the fungus (mm) after 7 days at 22 C in an in vitro challenge assay. Isolate selection was based on the percent frequency of occurrence of individual species in each specific community sampled.

Table 2. Relative effectiveness of field applied foliar treatments in inhibiting potato late blight leaf infection and disease development, following inoculation of potato leaf tissue^a with Phytophthora infestans.

a Based on the examination of sample of 10 whole leaves per treatment replicate and 4 replicates inoculated with a 20uL droplet containing a suspension of P. infestans zoospores at a concentration of 1000 spores per ml.
b Manzate® 75 DF.
c Percent data arcsine transformed data for statistical analysis, untransformed numbers are presented.
d Numbers in rows followed by the same letter are not significant different using a protected Least Significant Difference test (P= 0.05).

Author Locations and Affiliations
1. PEI Department of Plant Health Research & Diagnostics, Prince Edward Island Department of Agriculture, Fisheries, Aquaculture and Forestry, P.O. Box 1600, Charlottetown, PE C1A 7N3
2. Organic Agriculture Centre of Canada, Nova Scotia agricultural College, Box 550, Truro, NS B2N 5E3
3. Eric C. Robinson Inc./Ingleside Farms, P.O. Box 1, Albany, PE COB 1AO


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