Causal Organism: Podosphaera leucotricha
Description: The disease infects leaves, blossoms, Green shoot, and the fruit. On leaves, the fungus can appear as felt-like patches or as a solid mat on the surface, particularly in undersurface area of the leaf. Initial infections on the underside of the leaf may cause chlorotic patches or as a solid mat on the surface,, particularly in undersurface area of the leaf. Initial infections on the underside of the leaf may cause chlorotic patches or spots on upper side of the leaf. This symptom, however, is not unique to powdery mildew so inspection of underside of the leaf is necessary to confirm that powdery mildew is the cause of these symptoms. Under favorable environmental conditions, the disease spreads over the entire leaf and progresses down the petiole on to young, green shoots. Infected leaves tend to crinkle, curl, or roll upwards along the edges giving them a narrow appearance. If, the infection is severe, infected leaves will usually drop prematurely during the summer.
Infected terminal shoots are stunted and the leaves along the shoots appear as described above. Shoot infections are typical result of overwintering infections of the fungus in buds. When terminals push in early spring, the fungus grows along with the new succulent growth and infects this tissue immediately. These shoots may be killed outrightly during spring, or may survive throughout the season and die in late fall or winter. The initial growth of the fungus on newly infected twigs appears powdery white but eventually turns to dark brown. Small black fruiting bodies, called cleistothecia, can appear in the mycelia mat and function as source for sexual reproduction and the eventual production of ascosporoes.
The blossoms petals, sepals, receptacles, and peduncles may become infected and covered with the fungus. Blossom infections are less common but are important because infected blossoms will either fail to set fruit or produce small, stunted and/or russetted fruit. These fruit are unmarketable for consumption.
The powdery mildew fungus overwinters as mycelium in infected buds or as cleistothecia on the surface of infected twigs. Infected terminal buds are more susceptible to winter injury than healthy buds. Bud break during spring is 5-8 days later than healthy buds, and are more susceptible to spring frost than healthy buds.. In fact, healthy buds may survive at temperature 2-10°C older than infected buds. As a result, many infected buds will not survive through a cold winter. This is important to keep in mind because a hard winter can dramatically reduce both disease pressures and the need for control measures during the subsequent season. The survival rate of infected buds is less then 5 % when temperatures drop below -24°C and although not well studied, it appears that temperatures around -12°C will kill the mycelium in infected buds and allow the bud to produce healthy leaves. The cleistothecia, which form the sexual stage of the fungus and produce ascospores, apparently do not play an importance role.
Mycelia in infected buds produce conidia to initiate primary infections and these infections can be found as early as the tight cluster stage. There is often an abundance of susceptible tissue for the conidia to infect because infected buds break later then healthy buds. Conidia are disseminated by wind and can infect young leaf tissue, blossoms, and fruit. Leaves become increasingly resistant to infection as they age and become nearly immune once they have matured, although infection can occur through injuries on older leaves. Symptoms may develop as early as 5 days after infection. Numerous secondary cycles can occur under favorable condition and like many powdery mildews, cooler temperatures rather than relative humidity drive early secondary infections. Like other powdery mildews. Leucotricha exhibits a diurnal periodicity in that the highest concentration of airborne conidia is found from mid-day to early afternoon. Infections that result in fruit russet occur primary during the pink stage of bud development.
Apparently, infection of lateral and fruit buds occurs within 1 month after they are formed. The infections remain latent until bud break the following spring where they will serve as the initial source of inoculum.The lateral buds are susceptible to infection longer than the terminal buds, however, it is the terminal buds that are the likely source of overwintering of the fungus as infection can be grater than 50 percent by terminal bud set.