Oral Presentation Science Protecting Plant Health 2017

Do ascospores play a role in the epidemiology of citrus black spot? (4021)

Nga Tran 1 , Andrew Miles 1 , Ralf Dietzgen 1 , Andre Drenth 1
  1. Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, Brisbane, QUEENSLAND, Australia

Citrus black spot caused by the fungus Phyllosticta citricarpa is characterised by fruit blemishes and premature fruit drop, resulting in significant economic losses in tropical and subtropical growing areas worldwide. Although the disease has been known of for over 100 years, several aspects of its biology and epidemiology remain unclear, hindering development of effective disease management strategies. To improve our understanding of the disease cycle, we aim to address the following questions: i) How does the fungus undergo sexual reproduction? ii) What is the role and seasonal dynamics of ascospore release in the field? and iii) When do key infection events occur? We have recently been successful in producing ascospores in pure culture by developing a novel mating technique involving pairing of isolates of complementary MAT idiomorphs. Being able to produce ascospores in vitro has enabled us to experiment with ascospore inoculations to investigate their pathogenicity. In addition, volumetric spore trapping combined with a staggered fungicide application trial was used to determine the pattern of ascospore release, timing of infection, and the link between these two factors under field conditions. Field trial results have shown that while the peak of ascospore release was in February, the main infection events occurred in October and November, suggesting that peak ascospore release does not correspond with peak infection. Our study has confirmed heterothallism in P. citricarpa, but the frequency of successful mating between idiomorphs in the field, and its relationship with infection in orchards warrants further investigation. Future studies will investigate changes in fruit susceptibility and effects of environmental factors on infection to better understand the disease development and epidemiology of citrus black spot.