Dieback caused by Colletotrichum spp. is an emerging disease in California citrus groves. A large-scale survey with emphasis on seasonal variations of latent infections was conducted throughout citrus orchards in Fresno, Kern, and Tulare counties in 2019 and 2020. Latent infections on citrus leaves and twigs varied markedly between years. Isolates of Colletotrichum spp. were obtained from asymptomatic tissue and two groups were formed based on colony and spore morphology. The morphological groups were further identified based on multigene sequence analysis using the DNA regions ITS1-5.8S-ITS2, TUB2, and GAPDH. Results revealed that isolates belong to two phylogenetic species, C. gloeosporioides and C. karstii, being C. karstii more frequently isolated. Representative isolates of each species were further selected and characterized based on the response of physiological variables to temperature. Both species had similar optimum growth temperatures but differed in maximum growth rates, with C. gloeosporioides exhibiting a greater growth rate than that of C. karstii on media. Pathogenicity tests on citrus trees demonstrated the ability of C. gloeosporioides and C. karstii to cause lesions on twigs and no differences in aggressiveness. A fungicide screening performed in this study determined that the DMI fungicides were the most effective in reducing the mycelial growth of C. gloeosporioides and C. karstii. The QoI fungicides showed a remarkably inhibitory impact on spore germination of both species. On average, C. karstii was more sensitive to the DMI fungicides than C. gloeosporioides. The findings of this study provide new information to understand the Colletotrichum dieback of citrus.
Colletotrichum species infect temperate fruit trees and symptoms can occurin various parts of the plant such as twigs, leaves, fl owers, and fruits. The damage caused by these diseases is severe in Brazilian subtropical conditi ons, where a high primary inoculum potential is present. This review addresses e ological and epidemiological studies of Colletotrichum spp. in persimmon, apple, peach, and grapevine crops. General aspects of the management of the diseases under subtropical conditi ons are also discussed. Regarding etiology, new taxonomic classifications of species, brief history of nomenclature, phylogenetic studies for species identification and survey of species reported as causal agents of diseases were reviewed. Regarding epidemiology, favorable conditions for the pathogen and the stages of the disease cycles were addressed. For persimmon and apple crops, a more in-depth discussion about the diseases was held. Two disease cycles, in the form of illustrative fi gures were proposed: Colletotrichum spp. x persimmon and Colletotrichum spp. x apple tree. The cycles were based on literature and on epidemiological studies developed by our research group. The etiological and epidemiological studies reported here provide support for integrated management strategies for the diseases that are causing damage to these fruit trees in Brazil and worldwide
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