By Sam Markell
Importance. Powdery mildew can be a very important disease when conditions are favorable. It reduces seed size and can sharply reduce yields if it develops during early to mid-pod development. Late planted peas are at greater risk for yield loss than early planted peas. Most commercial field pea varieties grown in North Dakota are susceptible to the disease, but there are some varieties that are resistant (immune).
Symptoms. Powdery mildew causes white powdery fungal growth over all above-ground parts of the plant. The disease usually begins as small discrete white tufts. Once it appears, the disease can spread very fast and the white fungal growth can quickly cover entire leaves and other green tissues. As the disease is developing, the white fungal growth can be easily rubbed off, and the tissue underneath may appear normal or slightly yellowed. As the disease progresses, black specs (fungal reproductive structures) often develop within the white fungal growth, and the peas take on a bluish color.
Disease Cycle and Development. The pathogen overwinters in small black reproductive structures that release aerial spores in the spring/summer. Powdery mildew infection and development is favored when dry, warm weather are accompanied by nights that are cool enough for dew to develop. The disease can develop very rapidly when environmental conditions are favorable.
Management. Fungicide applications are highly effective against powdery mildew but it is critical to apply them before an epidemic begins; apply at either the first appearance of trace levels of powdery mildew in the lower canopy or on the basis of perceived risk if weather conducive to the disease occurs during bloom and pod-fill. Very little fungicide efficacy testing has been conducted for this disease on field peas, but the fungicides Proline, Quadris, Headline, and Priaxor (with triazole [DMI / FRAC 3], strobilurin [QoI / FRAC 11], and/or SDHI [FRAC 7] active ingredients) have shown efficacy in the relatively small number of field trials that have been conducted evaluating fungicides for control of this disease.