Biopesticides (Seem to) Perform Poorly Against Black Spot on Kale

From Clemson Plant Pathologist Tony Keinath.

As part of a USDA Organic Transitions Program grant, we tested 10 biopesticides on organic kale ‘Oldenbor’ to see if any product controlled Alternaria leaf spot, a.k.a. black spot. Unfortunately, none look promising as no biopesticides reduced black spot compared to the nontreated control based on repeated greenhouse trials and one field trial in spring 2022. This difference is the minimum of what I look for before recommending a biopesticide to manage disease.

Healthy and diseased leaves harvested from a plot treated with a biopesticide. Note that the healthy leaves are the smaller, younger leaves, and the diseased leaves are the older, larger leaves.

A few other differences among treatments in the field trial might be important to note. The weight of healthy cropped leaves was greater with ProPhyt and Sil-Matrix, a silicon product consisting of 29% potassium silicate, than with Stargus, Double Nickel, and Milstop. In addition, ProPhyt and Sil-Matrix might slow or restrict spread of Alternaria and keep it off the middle-aged leaves. (Note that young leaves in the upper one-third of the plant rarely have black spot.) Since ProPhyt is a popular alternative fungicide used to manage downy mildew on collard, applications in the fall might also help manage Alternaria. We will be following up on these results with more field trials starting this fall.

Effect of biopesticide applications on marketable yield of kale in the presence of black spot. Note that all treatments were inoculated with Alternaria.

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