Rusty Spot on Peach

Most tree fruit pathologists consider rusty spot to be a minor peach disease in Mid-Atlantic orchards, but it can be observed when there are adjacent blocks of apple trees with active powdery mildew infections nearby. When I have observed rusty spot in Pennsylvania peach orchards, its presence can often be linked to a breakdown in the grower's protectant spray program, a mildew-susceptible peach variety, an adjacent active powdery mildew infection (in an apple block), and favorable weather conditions.

Rusty spot initially appears as a white powdery spot that becomes hard and leathery before turning a rusty brown in color on mature fruit at harvest. The pathogen that causes rusty spot on peach trees overwinters as mycelium on dormant peach buds or in nearby apple blocks. As infected shoots emerge in the spring, conidial spores are produced and are carried by the wind and rain to peach fruit. Conidial spores can also be produced on infected host plants outside of the peach orchard (apple trees).

Peach fruit are susceptible to infection when the fruit are young and soft. As the pit hardens, peach fruit become more resistant to rusty spot infection. Rusty spot is most severe under moderate temperatures and high humidity. Rusty spot spores germinate at relative humidities between 43 to 100%. Free moisture, however, is not required for rusty spot infection to take place. The optimum temperature for the germination of rusty spot or spores on peach trees is 66 to 72°F.

Rusty spot can be prevented in peach orchards by avoiding powdery mildew-susceptible varieties (Loring) and through the management of powdery mildew in adjacent apple orchards. If powdery mildew-susceptible peach varieties are planted, protectant fungicidal sprays will be necessary for management. Protectant fungicides to prevent rusty spot should be applied at petal fall, shuck split, first cover, and second cover. Peach fruit will develop resistance to rusty spot as the pit hardens. Fungicides that have activity on peach scab and brown rot will frequently control rusty spot in peach orchards.