Powdery mildew of peach occurs worldwide, but is most damaging in semi-arid growing areas. This means that in some years in California the disease can be severe. The disease can be caused by several different species of powdery mildew fungi that commonly occur on Rosaceous plants.

Historically, three species have been reported on peach with Podosphaera pannosa (formerly Sphaerotheca pannosa) being the most important one. Podosphaera leucotricha is less common and P. clandestina has been reported on peach seedlings in the eastern United States. More recently a fourth species we have identified a fourth species P. tridactyla in the Central Valley. Fruit infections can be caused by P. pannosa, P. leucotricha and P. tridactyla resulting in the most economic damage. Stem and leaf infections are important sources of overwintering and secondary inoculum, respectively.

The susceptibility of peach and other stone fruit crops varies greatly among cultivars. The eglandular (without glands at the leaf base) peach cultivars are more susceptible than the glandular ones. Furthermore, in some cultivars, tissues also vary in their susceptibility with fruit being more or less susceptible than leaves, depending on the mildew species involved and maturity of host tissue.

Leaves, buds, green shoots, and fruit are commonly attacked by most powdery mildew fungi, but flower infections are rare. Symptoms include circular, white, web-like colonies that become powdery once masses of asexual conidia are produced in chains on all tissues. Leaves may then curl or become stunted. Severe infections commonly cause leaf chlorosis, necrosis, and leaf drop.

For mildew caused by P. pannosa, fruit are susceptible from the early stages of development until pit-hardening on peach. For P. tridactyla, fruit may be susceptible for extended periods but this is not completely studied. White circular spots may enlarge, coalesce, and cover large areas of the fruit. Based on indirect evidence, P. leucotricha (mainly an apple pathogen) presumably is involved in causing another powdery mildew symptom on peach fruit known as “rusty spot”.

With this disease, small, circular, orange-rusty lesions develop on the fruit that enlarge and may cover the entire fruit. No symptoms occur on leaves and stems. Lesion development has been related to rapid fruit growth. Infections for all powdery mildew species usually result in some deformation of the fruit surface with depressed or slightly raised areas. Secondary infections caused by other fruit decay fungi may also occur in necrotic mildew lesions.

Disease cycle. In the spring, newly developing leaves become diseased as they emerge from infected buds. When overwintering spore cases (chasmothecia) are present, ascospores are released that serve as primary inoculum. Because roses are an important host for the P. pannosa pathogen where the disease is not always managed, diseased roses can be major contributors to the development of epidemics of peach powdery mildew.

Secondary infections by the wind-disseminated, asexual conidia occur throughout the growing season. Conidia germinate between 2 C and 37 C, with an optimum of 21 C. Conidia can germinate in free water and at relative humidity of 43 percent to 100 percent. Excessive durations of wetness will kill conidia of powdery mildew fungi. During periods with warm, humid conditions the disease can quickly develop into an epidemic.