Characteristics of Walnut Powdery Mildew and a Detailed Guide to Its Prevention and Control

  1. Distribution and Occurrence Characteristics It is one of the common and important diseases in walnut-producing regions across our country. The disease severely affects the yield and quality of walnuts in the current year and the tree vigor in the following year, causing great damage. The pathogen overwinters as cleistothecia on fallen leaves or diseased shoots. The following spring, as temperatures rise and rain occurs, the cleistothecia absorb water, swell and rupture, releasing ascospores that spread with airflow to young buds, shoots and leaves for primary infection, with disease occurring in July to August. After infection, conidia are produced multiple times on the lesions for secondary infection. In autumn, small granules, namely cleistothecia, are again produced on diseased leaves and overwinter with the fallen leaves. Warm and dry conditions, excessive nitrogen fertilizer, insufficient potassium fertilizer, and inadequate shoot development make the disease prone to occur, and young trees are more susceptible than large trees.

The picture shows: walnut

  2. Pathogen and Damage Symptoms It is a fungal disease, and the pathogens causing it are two species: Phyllactinia guttata and Phyllactinia juglandis. It damages leaves, young buds and new shoots, causing early defoliation and even death of seedlings. In the early stage of the disease, leaves fade green or develop yellow spots; in severe cases, leaves become distorted and wrinkled and fall off early, young buds sprout but cannot expand leaves, and thin white powdery layers appear on the upper or lower surface of the leaves. Later, brown or black granules appear in the white powder, or the powdery layer disappears leaving only small black granules, namely the cleistothecia of the sexual stage of the pathogen. When seedlings are damaged, the plants become stunted, the tips die, and even the whole plant dies.

  3. Prevention and Control Methods

  ① After harvest, remove diseased and residual branches and leaves and burn them in a centralized manner to reduce the source of infection.

  ② Chemical control. In the early stage of the disease, spray with 0.2-0.3 Baume lime sulfur mixture. In summer, spray with 50% thiophanate-methyl wettable powder at 800-1000 times dilution or 25% triadimefon at 500-800 times dilution, with the latter giving better control results.