breeding plots when I took a few minutes to photograph downy spot, a minor pecan disease not often seen in well tended groves. This disease popped up in the breeding plot for two reasons. First, we don't apply fungicides to the breeding block to enable us to check for scab resistance. And second, 15 inches of rain over 4 weeks in mid-summer provided great conditions for disease spread.
By the time I photographed downy spot, the disease was in in advanced stages of development. Looking at the underside of a leaflet with the sun streaming from above, it is easy to see the irregular shaped lesions of downy spot. Each lesion appears yellow with dark patches at the margins and within the lesion (photo at right).
Downy spot is caused by the fungus, Mycosphaerella caryigena, which overwinters on fallen leaves from the previous season.
Downy spot is usually controlled when fungicides are applied for the control of pecan scab.