Ultrastructural biology and pathogenesis of twig blight disease (TBD): newly identified major fungal disease of citrus in the Philippines

This study tried to investigate the detailed morphological characteristics of signs and symptoms of the diseases as well as the ontogenetic study of citrus twig blight in the Philippines. The ultimate objective of the study is to investigate the ultrastructural biology and different stages of pathogenesis of H. haematoccoca infecting citrus plant. A twig blight disease of citrus (Citrus reticulata Blanco) has been recognized for about five years in the Philippines. Results from in-vivo inoculation studies indicate that citrus branches was incited by homothallic strains of Haematonectria haematoccoca (anamorph: Fusarium solani) causing twig blight disease. The symptoms of the fungus disease appear as a pink or salmon-colored, smooth mycelial mat on branches of its hosts. Mycelial coil as lateral branches of the vegetative hyphae formed when ascocarp developed by H. haematoccoca. The multinucleate ascogonia were surrounded by pseudoparynchymatous envelope. Development of an apical meristem produces cells that elongate downward into the centrum during ascocarp development with long, filamentous apical paraphyses. Four nucleate cells were then observed due to the conjugation of two nuclei in the cells. The diploid nucleus was gradually forming 2 and 4 nucleate stages right after meiosis and immediately followed by mitotic division in order to produce an 8 nucleate ascus. Ultrastructure study revealed that the ascus of H. haematoccoca was unitunicate in nature which adhered closely throughout the life of the ascus, and the spores are released through a terminal pore, slit, or hinged cap (operculum). Hence, H. haemotoccoca considered as unitunicate operculate asci. Ascospores and conidia both had uninucleate cells germination occur 24 hours after dispersal. The conidiophores with lateral hyphae produced macroconidia 3 days after the spores have been germinated. It produces two types of conidia, a macro and micro conidia. The fungus produces hyaline, two-celled or didymospore, ovoid to oblong, the conidia is about 20-30 um in length and 10-15 um in width. Lateral branch of hyphae was observed 6 hours after spore was germinated. Appressorium was visible and developed 5 days after inoculation. The cells of the lenticel were disintegrated 8 days after inoculation. Full colonization and production of asexual spores covering the entire surfaces were observed 15 days after inoculation. Conidia were observed to mature 18 days after inoculation and released at 19 days after inoculation. Asexual and sexual reproduction completed in 18 and 27 after inoculation respectively. This is a first report of twig blight disease of citrus caused by H. haematoccoca in the Philippines