Response of two fungi in the apple sooty blotch complex to temperature and relative humidity

Peltaster fructicola and Leptodontium elatius, two of the causal fungi of apple sooty blotch, responded differently to temperature and relative humidity in vitro, Conidia of L. elatius germinated from 12 to 32 degrees C at relative humidities greater than or equal to 97%, whereas conidia of P. fructicola germinated from 12 to 24 degrees C at relative humidities greater than or equal to 95%. Germination of conidia of L. elatius was optimum at 32 degrees C and 99% relative humidity compared with 24 degrees C and 97 or 99% relative humidity for P. fructicola. When L. elatius and P. fructicola were grown in Parafilm culture, sporulation was greatest at relative humidities of 97 to 99%. In agar culture, mycelia of L. elatius expanded radially from 12 to 32 degrees C, and that of P. fructicola at 12 to 28 degrees C. Mycelia of P. fructicola did not survive exposure for 7 days or more to temperatures greater than or equal to 32 degrees C. Mycelial growth was inhibited at relative humidities <95% for both fungi and no growth occurred at 88% relative humidity. Conidia of P. fructicola were more sensitive to air drying than were those of L. elatius. Conidial viability of P. fructicola was reduced significantly after 8 h of air drying and nearly completely inhibited after 12 h. Conidia of L. elatius required 24 h of air drying before a significant reduction in conidial viability was observed. These results support the hypothesis that environmental factors influence the temporal and geographical distributions of the fungi associated with the apple sooty blotch disease.