N. T. Tran

Sexual Reproduction in the Citrus Black Spot Pathogen, Phyllosticta citricarpa

Citrus black spot (Phyllosticta citricarpa) causes fruit blemishes and premature fruit drop, resulting in significant economic losses in citrus growing areas with summer rainfall across the globe. The mating type locus of P. citricarpa has recently been characterized, revealing the heterothallic nature of this pathogen. However, insight into the occurrence of mating and the impact of completing the sexual cycle of P. citricarpa was lacking. To investigate the occurrence and impact of sexual reproduction, we developed a method to reliably, and for the first time, produce ascospores of P. citricarpa on culture media. To demonstrate meiosis during the mating process, we identified recombinant genotypes through multilocus genotyping of single ascospores. Because the process of fertilization was not well understood, we experimentally determined that fertilization of P. citricarpa occurs via spermatization. Our results demonstrate that P. citricarpa is heterothallic and requires isolates of different MAT idiomorphs to be in direct physical contact, or for spermatia to fulfill their role as male elements to fertilize the receptive organs, in order to initiate the mating process. The impact of mating on the epidemiology of citrus black spot in the field is discussed.

Does Fungicide Run-off From Citrus Delay Leaf Litter Decomposition?

Leaf litter is a major inoculum source for citrus diseases such citrus black spot caused by Phyllosticta citricarpa, and greasy spot caused by Mycosphaerella citri. In order to reduce this inoculum source, the efficacy of urea, dolomitic lime, a commercial compost accelerator, and an organic mulch, was assessed for enhanced leaf decomposition and reduction in sporocarps. However, due to the potential for run-off from high volume fungicide applications to disrupt leaf decomposition and microbial antagonism, the amendments were compared with and without simulated fungicide run-off. Mature green leaves of Citrus sinensis were removed from trees and placed inside mesh bags before being pinned to the orchard floor. The amendments were applied, and then simulated run-off from a typical citrus black spot fungicide program (copper, mancozeb, azoxystrobin) was applied. Leaf degradation was assessed every 2-3 weeks by visual ratings and dry weight. No direct effects on sporocarps could be observed due to insufficient infection. The results showed that the organic mulch was the most effective at enhancing decomposition, while there was significantly (P < 0.05) less decomposition in the presence of fungicide run-off.