Studies On Identification And Control Of The Causal Organism Of Black Spot, A New Ligustrum×Vicaryi Disease Wang Tan

Ligustrum×vicaryi is known as an important ornamental landscape tree. A new leaf spot disease (black spot) of Ligustrum x vicaryi severely occurred in recent years in Nanyang city, Henan province, resulting in heavy defoliation. To charify the causal organism of the disease, and to establish effective control measures against the disease, isolation and purification of the pathogen were conducted. The pathogenicity of two representative isolates was confirmed based on Koch’s postulates. Partial biological characters of the pathogen were investigated. The identification of the pathogen was carried out based on its morphological characters and rDNA-ITS (internal transcribed spacer) sequences. In addition, both chemical and biological control measures were tested in fields. The main results were summarized as follows:1. The pathogen was identified as Alternaria alternata (Fr.:Fr.) Keissler according to its morphology, pathogenicity and sequence analysis of rDNA-ITS. The optimum temperature for mycelial growth was25-28℃. The pathogen could grow at pH4-9. Of the12utilizable carbon sources tested, xylose and glucose were most suitable for vegetative growth of the pathogen. Of the8utilizable nitrogen sources tested, DL-alanine and glicin were the best for vegetative growth of the pathogen. The productivity of extracellular enzymes by the pathogen was tested. The results showed that the pathogen produced lipase, cellulase, amylase, but not prolease.2. Totally290endophytic bacterial isolates were recovered from healthy Ligustrum×vicaryi plants. Of which,46isolates had inhibitory activity against vegetative growth of A. alternana, contributing to15.9%of the isolates obtained. Both endophytic isolates A71and C108showed stronger inhibition on the growth of the pathogen. On the basis of morphological, physiological and biochemical characteristics as well as16S rDNA sequence analysis, the isolate A71was identified as a member of Burkholderia cepacia. The optimum pH and temperature were6-9and25-30℃for the growth of isolates A71and C108, respectively. Isolate A71could use all test carbon sources, but isolate C108could not use xylose, D-fructose, xylitol, mannitol, L-pectinose as carbon sources. Both isolates A71and C108could use all test nitrogen sources for their growth, including beef extract, yeast extract, KNO3and (NH4)2SO43. The results of toxicological test showed that the EC50value of75%chlorothalonil WP was2.34mg/mL and the EC50value of70%mancozeb WP was2.23mg/mL. The results of field trials indicated that the mixed application of70%mancozeb WP (1400g a.i./ha) plus75%chlorothalonil WP (750a.i./ha) had the highest control efficacy of64.96%against black spot disease of Ligustrumx vicaryi.The efficacies of the endophytic bacterial isolates A71and C108were54.1%and54.9%, respectively, in controlling the disease.