What was it all about?

Citrus black spot disease is a fungal disease estimated to cost Australian growers around $80 million annually through export restrictions, fungicide applications and fruit damage.

The need to manage the disease in Australia, and also reduce its spread as an emerging disease in Florida, provided an ideal opportunity for collaboration across countries. The project involved teams in both Queensland and Florida, and ran from 2014 to 2018. The work was funded through the Hort Innovation Citrus Fund using contributions from the Citrus Research and Development Foundation, the University of Florida and the Queensland Alliance for Agriculture and Food Innovation (the University of Queensland), plus contributions from the Australian Government.

Citrus black spot primarily arises from the production of airborne ‘ascospores’ that are formed in the citrus leaf litter in orchards. The ascospores then infect young susceptible fruit, resulting in black spot symptoms when fruit matures.

The project investigated areas:

• Leaf litter reduction in orchards
• Sporulation patterns of the fungus
• Sources of citrus black spot resistance within citrus germplasm.

The research team found that applying an organic mulch layer over leaf litter was the best way to prevent the release of ascospores, although this practice would be expensive and time consuming in orchards.

Reducing fungicide run-off was the next most effective approach to reducing black spot, since fungicide run-off from high-volume spray application methods affects leaf litter degradation. Other leaf litter amendments were not highly beneficial.

Through monitoring leaf litter in orchards across the season, the researchers found another species of fungus, a non-destructive organism known as Phyllosticta capitalensis, that inhibits citrus black spot development. The researchers concluded that promoting P. capitalensis in orchards may be beneficial, but were unable to find a fungicide that allows only the beneficial species to flourish.  

At the project’s Bundaberg Research Facility, various citrus accessions were tested for susceptibility to citrus black spot in the hopes of finding desirable varieties with resistance to the disease. One, the K15 pomelo, was found to have some resistance, which has opened up an opportunity in breeding for resistance to citrus black sport in citrus in the future.