Fungal-Infected Weeds: A Potential Source of Leaf Spot Disease in Rubber Trees from Southern Thailand

The rubber tree (Hevea brasiliensis) is an economically important crop in Thailand. Severe defoliation caused by emerging diseases has been reported to substantially reduce rubber yields during the leaf fall phase. The classical disease dispersal patterns of fungi in rubber tree plantations might be derived from weeds in adjacent fields. However, this hypothesis remains untested. Therefore, in this study, we collected and isolated fungi from symptomatic weed samples in rubber tree plantations in Krabi Province in southern Thailand. We found that Parameria sp. were dominant, showing the development of conidiomata on leaves. A total of 25 symptomatic Parameria sp. leaves were collected and tested for their pathogenicity on rubber tree leaves. The tests produced six fungal isolates, WC001, WC002, WL001, WL002, WN001, and WN002, that caused spots on the rubber tree leaves similar to those observed on the weeds. Morphological characterization revealed that fungal isolates WC001 and WC002 were Colletotrichum sp., WL001 and WL002 were Lasiodiplodia sp., and WN001 and WN002 were Neopestalotiopsis sp. Multigene phylogenetic analyses of combined act, gapdh, ITS, and tub2 regions identified WC001 and WC002 as Colletotrichum siamense, while analyses of ITS, tub2, and tef1-α regions identified WL001 and WL002 as Lasiodiplodia brasiliensis and WN001 and WN002 as Neopestalotiopsis cubana. The occurrence of fungal diseases in rubber trees is significantly associated with leafy weeds in and around rubber tree plantations that could constitute reservoirs of fungal pathogens. The strategies used to control weeds have to be further considered in the future.