Diseases of pecan (
Powdery mildew
Powdery mildew of pecan is caused by the fungus Microsphaera alni (also called Microsphaera penicillata). The fungus infects the shucks of the nuts (photo 1). It also may be found on new leaves, especially those of volunteer pecan seedlings, underneath mature trees. Mature leaves are not affected. The powdery white to gray growth on the surface of shucks appears quickly orchards in mid June to late June. Infections probably originate from spores that are produced from the fungus that has over-wintered in bark of pecan trees. It occurs routinely in orchards in the Green Valley area but is rare in other locations in Arizona.
Like all powdery mildews, pecan powdery mildew is an obligate parasite and will only grow and reproduce on its host plant. It has a very restricted host range. The Microsphaera species on pecan in Arizona may also infect the closely related Arizona walnut (Juglans major), but this not not been confirmed.
Studies on nut weight and color indicate that infection does not affect nut quality. In experiments comparing uninfected nuts (treated with an efficient fungicide) and of infected nuts (not treated), there was no difference in weight or quality of nuts (http://ag.arizona.edu/pubs/crops/az1178/az1178-12.pdf). Therefore, control measures are not recommended.
1 
March 30, 2007