Effect of autumn application of urea on saprotrophic fungi in off-season leaf litter of sour cherry and evaluation of fungal isolates to reduce primary inoculum of Blumeriella jaapii

Abstract

The effect of autumn application of urea on the activity and composition of saprotrophic fungi associated with sour cherry (Prunus cerasus) leaf litter infected with the leaf spot pathogen, Blumeriella jaapii, was studied under orchard conditions from the time of leaf fall to the release of ascospores, the following spring. The ability of treatment of infected leaves post leaf fall with urea or with inoculum of selected fungal strains to reduce primary inoculum (ascospores and winter conidia) of B. jaapii in spring was evaluated. Fungal activity was measured enzymatically by quantifying β-N-acetylgly-cosaminidase and fungal composition and frequency were evaluated following isolation at low temperature of mycelia embedded in leaf particles. Activity of the general fungal flora increased in both the urea and water treatments throughout the experimental period and a significantly higher level of activity was detected in urea-treated leaves up to 79 days after treatment. Most of the 2,146 fungal isolates recovered from the leaf litter during November-May could be placed in the following six groups: Phoma macrostoma (52.7%), other Pho-ma spp. (12.3%), Cladosporium spp. (13.8%), Alternaria and Ulocladium spp. (5.4%), Epicoccum purpurascens (2.9%) and Fusarium spp. (1.6%). These groups were isolated from the leaf litter throughout the experimental period and the effect of urea on fungal composition in the litter was very limited. Of these groups only the frequency of recovery of E. purpurascens was higher from urea than from water-treated leaves and this significant difference was maintained throughout the 115 days of the experimental period. Phoma macrostoma var. macrostoma, Cladosporium sp., Ulocladium chatarum, Epicoccum purpurascens and Fusarium lateritium var. lateritium were evaluated and compared with urea to reduce primary inoculum of B. jaapii. Urea reduced primary inoculum of B. jaapii in the spring by 77% and of the fungal strains, especially Cladosporium strain MB167 was found to be equally effective.