Because there’s so much information to share in a short amount of time, at the most recent Great Lakes Expo, Kari Peter, Ph.D., associate research professor of tree fruit pathology at Penn State, combined her presentation on brown rot and rusty spot in stone fruit. We shared her research on the former in the April edition of Country Folks Grower; below, see what she had to discuss when it comes to rusty spot.

Rusty spot (possibly caused by Podosphaera leucotricha, the fungus that leads to apple powdery mildew, which results in symptoms like white powdery growth on leaves, shoots and fruit) is being found on stone fruits. It causes rust-colored spots on the fruit, potentially making them unmarketable.

To thrive, rusty spot requires three key factors – the pathogen (spread via fungal spores), a susceptible host and a hospitable environment. Peter said the pathogen most likely overwinters in peach and apple buds, making it hard to control culturally since it lives in the buds.

It thrives in temperatures of 50º – 77º F and humidity greater than 70%, but leaf wetting is an infection deterrent.

The full disease cycle is currently unknown, but spores are available starting at petal fall and continue spreading for approximately one month after shuck split (until the pit hardens).

“The same fungus that causes powdery mildew on apples causes powdery mildew/rusty spot on peaches and nectarines,” Peter said, noting that the fungus only affects the fruit of peaches and nectarines, not other plant parts.

To manage rusty spot, Peter suggested applying fungicides at petal fall, shuck split, first cover and second cover. She said effective fungicides include sulfur, Rhyme®, FRAC 3s (Rally®, Cevya®, Inspire Super™, Luna® Flex), FRAC 7s (Miravis®, Fontelis®) and FRAC 11s (Flint® Extra, Luna® Sensation, Merivon®).

Peter cautioned growers to be mindful of what they apply from bloom and to practice fungicide resistance management.

While there is “a spectrum of susceptibility” when it comes to varieties, Peter mentioned some standouts: peaches/nectarines with very low susceptibility to rusty spot include ‘Blake,’ ‘Gloria,’ ‘Harrow Beauty,’ ‘Saturn,’ ‘Sugar Lady’ and ‘Sugar May.’ On the other end of the spectrum, varieties with very high susceptibility include ‘Autumnglo,’ ‘Jerseyqueen’ and ‘Rio-Oso-Gem.’

However, to give all cultivars a fighting chance at avoiding rusty spot, Peter drove home the message of breaking that “disease triangle” – the pathogen, the host and the environment. This is challenging, she said, “since the disease cycle is unknown, but use the critical sprays – at petal fall, shuck split, first cover and second cover.” (She added that with cherry powdery mildew, growers should spray from shuck fall through harvest.)

There is a silver lining, though: the hotter summers growers have been seeing may be a good thing. Peter said higher temperatures don’t eliminate pathogens, but they do make them go dormant until they can spread again.

by Courtney Llewellyn