Symptoms
Rusty spot is recognized only on the fruit. The earliest symptoms
on young green fruit appear as small, orange-tan spots. This symptom is due to a
change in the color of the fuzz or hairs on the fruit. These first spots may
become noticeable three to four weeks after shuck fall. The discolored area
enlarges slowly, and the older discolored hairs begin disappearing, leaving a
fuzzless, smooth, center spot surrounded by a non-uniform band of orange to tan
hairs. Finally, the spots become quite spread out leaving brownish or reddish
centers of hard, smooth skin that appear somewhat like a bruise from a limb
rub.
Disease Cycle
Since the cause of rusty spot is not clear, the disease cycle is
unknown. Although most infection seems to occur from petal fall to one month
after shuck fall, some new spots on the fruit may continue to appear up to the
time of harvest. There does not seem to be any relationship between weather
conditions and the number of diseased fruit.
Disease Management
Choose varieties that are not susceptible to rusty spot. Spotting
on the fruit of some varieties appears to be due to the powdery mildew fungus.
Rio-Oso-Gem, Jerseyqueen, Jefferson, Washington, Redskin, and Loring often are
affected. Sulfur, Benlate, Topsin-M, or Funginex used at full labeled rates may
reduce rusty spot incidence.
