UC Riverside

Changes in pathogen populations, standard production practices, and regulatory restrictions have rendered current management strategies insufficient in pathosystems of strawberry and almond in California. New, improved and integrated strategies seek to reduce the economic impact of disease, reduce the risk of pathogen spread, and prevent development of pathogen resistance to chemical strategies.

Anthracnose crown rot of strawberry, caused primarily by Colletotrichum acutatum in California, is an important disease impacting nursery and fruit production. Pre-plant dip treatment with azoxystrobin failed during a recent disease outbreak, and QoI-resistance was confirmed. In evaluation of alternative treatments, the biofungicide natamycin was identified and was highly effective at reducing disease severity and mortality caused by QoI-sensitive or resistant isolates, and based on this work, was registered federally as a new dip treatment of nursery plants.

Angular leaf spot of strawberry, caused by Xanthomonas fragariae, has impacted export of California fruit due to quarantine restrictions. Pre-harvest chemical management is limited to copper, which is not very efficacious and can be phytotoxic. The bactericide amino thiadiazole was highly effective by itself or in select mixtures, and reduced disease incidence to low levels. Post-harvest fumigation with propylene oxide at select doses significantly reduced bacterial populations in infected leaflet tissues by at least 2.5-log compared with controls, with no phytotoxic effect to leaves or fruit. Together, this systems-management approach could reduce the risk of X. fragariae on fruit destined for export.

Bacterial spot, caused by Xanthomonas arboricola pv. pruni, is herein reported as a new disease of almond in California. Epidemiological studies demonstrated susceptibility of ‘Fritz’ almond over the spring season following inoculation of fruit and leaves, as well as flowers with subsequent disease development on fruit. Mummified fruit and associated spurs were identified as pathogen overwintering sites. Dormancy application of copper or copper-mancozeb significantly reduced disease in high-rainfall years. In-season bactericide application at petal fall or full bloom and petal fall also significantly reduced disease incidence, but was associated with phytotoxicity in some trials. Dormancy and in-season treatments of copper-mancozeb mixtures integrated with removal of mummified fruit are currently the best strategies for managing the disease.