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Poster: Biology & Disease Mgmt: Mycology


Nutritional and environmental effects on conidial germination and appressorium formation of Phyllosticta citricarpa, the citrus black spot pathogen
N. WANG (1), M. Dewdney (2) (1) Department of Plant Pathology, Citrus Research and Education Center, University of Florida, U.S.A.; (2) mmdewdney@ufl.edu, U.S.A.

Citrus black spot caused by Phyllosticta citricarpa has been present in Florida since 2010 and can reduce fruit yield and marketability. Conidial germination and appressorium formation (AF) are essential for successful infection by P. citricarpa, although the conditions required for this process are poorly understood. In this study, the effects of citrus juices, concentration, pH, various carbon and nitrogen sources, and environmental conditions were evaluated in vitro. All tested juices, especially Valencia (> 85%, P < 0.05), favored germination and AF, whereas sterile water rarely stimulated germination (< 1%). Juice quality analysis using Pearson’s correlation test revealed a moderate relationship between the Brix/Acid ratio and germination (r = 0.783, P < 0.05). The Valencia juice effect was concentration- and pH-dependent, and the maximum rate was reached in 1.5% juice with pH of 3.4. Most carbon, nitrogen, or complex sources did not favor germination or AF with exception of PDB, ammonium nitrate, and yeast extract. An incubation period of 18 to 24 h at 24°C were required for peak germination and AF. Further analysis of critical juice components using synthetic juice revealed sugars, salts, citric acid, and thiamine combined were most important for germination and AF (> 80%, P > 0.05). These results provide a better understanding of fungal biology and a robust and convenient system for further applications such as screening for efficacious fungicides.