What was it all about?

Black root rot, a severe disease of young avocado trees, is caused by soilborne fungal pathogens in the Nectriaceae family, with symptoms including black rotten roots, tree stunting and leaf wilt, often resulting in tree death within a year after transplanting into the orchard.

This project set out to investigate the species of fungi associated with black root rot and their relative ability to cause disease, with a view to developing a diagnostic test and some insights on how this disease may be managed.

The researchers began by collecting more than 120 fungal isolates from roots of avocado trees, and a further 30 from other trees including peanuts, blueberry, papaya and custard apple. 

Six main types of fungi were found and further analysis established some species names as well.

The varieties were assessed for the ability to cause disease. 

The main findings were…

• One species, Calonectria ilicicola, was the most aggressive of the fungi tested
• It was found to affect peanut, papaya and custard apple in addition to avocado
• Four species of Dactylonectria caused disease in avocado seedlings, although to a lesser extent.

The research team developed a diagnostic test that can detect the two damaging species in tree roots in around half an hour. It will be an invaluable tool for the avocado industry, especially for the ANVAS clean planting scheme, but will also be valuable for other industries impacted by these pathogens. The test has already been successfully employed in a sample of avocado roots sent to a commercial diagnostic laboratory.

Options for managing black root rot were explored and one fungicide, fludioxonil (Scholar), was effective at inhibiting growth of the key fungi in the lab and reduced root damage in seedlings.

The researchers believe that fludioxonil could be a powerful tool for growers in conjunction with careful post-planting care.

The outcomes from the study pave the way for future plantings of avocados to be free of black root rot. Researchers recommend discussions with the avocado and nursery industries to identify the best way to make screening services available to all nurseries.