A new disease of citrus has been found in the Central Valley of California. According to a report published in the University of California’s (UC) Topics in Subtropics newsletter, several growers and nurserymen in various orchards in the Central Valley first noticed the disease in 2013. Symptoms include leaf chlorosis, crown thinning, gumming, dieback, and in severe cases, death of young trees. The causal agents of this disease were identified as species of Colletotrichum, which are well-known pathogens of citrus and other crops that cause anthracnose diseases.
The most characteristic symptoms of this disease (see photo below) are the gum pockets, which appear on young shoots either alone or in clusters, and the dieback of twigs and shoots. Field observations indicate that symptoms initially appear during the early summer months and continue to express until the early fall. These symptoms were primarily reported on clementine, mandarin and navel orange varieties.
In order to determine the main cause of this disease, field surveys were conducted in several orchards throughout the Central Valley. Isolations from symptomatic plant samples frequently yielded Colletotrichum species. Morphological and molecular phylogenetic studies allowed the identification of two distinct species of Colletotrichum (Colletotrichum karstii and Colletotrichum gloeosporioides) associated with twig and shoot dieback.
Interestingly, these Colletotrichum species were also isolated from cankers in larger branches. Although C. gloeosporioides is known to cause anthracnose in citrus, a postharvest disease causing fruit decay, it has not been reported to cause shoot dieback of citrus. C. karstii, however, has not been reported previously in California citrus, and a research team is currently conducting field and greenhouse studies to determine the pathogenicity of this species in citrus.
At present, it is unclear how widespread this disease is in California orchards or how many citrus varieties are susceptible to this disease. Pest control advisors are monitoring citrus trees for the presence of the disease in the Central Valley (particularly clementine, mandarin and navel varieties) during the early summer months.
The newsletter article reports that the continuing research is a joint effort led by Akif Eskalen of UC Riverside in collaboration with Florent Trouillas of UC Davis. The research is focused on further understanding the biology of the fungal pathogens as well as factors influencing disease expression in order to develop management strategies against this emerging disease.
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