Identification and Epidemiological Features of Important Fungal Species Causing Sooty Blotch on Apples in the Northeastern United States

The sooty blotch and flyspeck (SBFS) complex causes blemishes on apples in humid, temperate growing regions worldwide. In contrast to flyspeck etiology, the many species of fungi causing sooty blotch (SB) have not been well studied. The first set of objectives in this study was to use PCR to identify SB species isolated from apples and selected reservoir hosts in the northeastern United States, and to identify patterns of species distribution on hosts and among sites. Results indicated that Geastrumia polystigmatis was the predominant species on apples, whereas Peltaster species were more common on reservoir hosts. Species distribution varied among sites. Phylogenetic analysis of 54 G. polystigmatis isolates revealed little genetic variability in the ITS region. The second set of objectives involved investigating the response of G. polystigmatis to changes in nutrition, temperature, heat stress, and relative humidity, and in vitro responses of G. polystigmatis and Peltaster fructicola to fungicides commonly used in orchards. Observation of growth on half-strength potato dextrose agar, malt extract agar, and 2% water agar revealed that mycelial growth of G. polystigmatis was thicker and more melanized in the presence of readily available carbohydrates. Temperature range experiments demonstrated that the optimum temperature for growth was approximately 24ºC. The fungus was able to survive exposure to 32ºC for at least one week, 37ºC for at least 48 hours, and 42ºC for at least 8 hours. Growth was optimum at 99-100% relative humidity. Isolates of P. fructicola were very sensitive to thiophanate-methyl, mancozeb, cyprodinil, penthiopyrad, fenbuconazole, and trifloxystrobin. Isolates of G. polystigmatis were sensitive to thiophanate-methyl and cyprodinil, but significantly less sensitive to all other fungicides than P. fructicola. The addition of salicylhydroxamic acid to trifloxystrobin significantly reduced growth of P. fructicola, but not that of G. polystigmatis. This study represents the first in-depth investigation into the identity of species causing SB in the Northeast, the basic biology of G. polystigmatis, and the fungicide sensitivities of G. polystigmatis and P. fructicola