Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/21/2002
Publication Date: 10/15/2002
Citation: SOMAI, B.M., DEAN, R.A., FARNHAM, M.W., ZITTER, T.A., KEINATH, A.P. ITS-1, ITS-2 AND RAPD ANALYSIS OF DIDYMELLA BRYONIAE AND RELATED PHOMA SPECIES ISOLATED FROM CUCURBITS. PHYTOPATHOLOGY. 2002. vol.92.p.997-1004 Interpretive Summary: Gummy stem blight is a destructive disease of cucurbit crops including cucumber, melon, and squash. The disease causes damage to leaves and reduces yields of infected crops. The occurrence of gummy stem blight results in significant loss of income by cucurbit growers. Although one fungus called Didymella is known to cause gummy stem blight, other fungi are often found on infected plants and these other fungi are sometimes confused with Didymella. In this research, samples of Didymella and the other fungi sometimes associated with gummy stem blight were compared at the gene or DNA level to determine if these different fungi are genetically related to one another and to help devise methods to readily identify the most destructive fungus. The results indicated that the different fungi could be categorized into four groups. Only one group, which contains the Didymella fungi, causes a high level of disease on cucurbits. The other groups cause little or no disease. Results of this research provide valuable information about the true causal agent of a serious cucurbit disease. This information will help agricultural scientists develop more effective measures that growers can use to minimize cucurbit crop losses due to the gummy stem blight disease.
Technical Abstract: Although Didymella bryoniae (anamorph Phoma cucurbitacearum) is the causal agent of gummy stem blight (GSB), other Phoma species often are isolated from cucurbit plants exhibiting symptoms of GSB. The molecular and phylogenetic relationships between D. bryoniae and these Phoma species are unknown. Isolates of D. bryoniae and Phoma obtained from cucurbits grown in various geographical locations within the United States were subjected to RAPD analysis and ITS-1 and ITS-2 sequence analysis to determine the molecular and phylogenetic relationships within and between these fungi. Using RAPD fingerprinting, 59 isolates were placed into four phylogenetic groups, designated RAPD group (RG) I, RG II, RG III, and RG IV. D. bryoniae isolates clustered into either RG I (33 isolates), RG II (12 isolates), or RG IV (one isolate), whereas all 13 Phoma isolates clustered into RG III. There was greater than 99% sequence identity in the ITS-1 and ITS-2 regions between isolates in RG I and RG II, whereas isolates in RG III, P. medicaginis ATCC 64481, and P. exigua ATCC 14728 clustered separately. On muskmelon seedlings, RG 1 isolates were highly virulent (mean of 71% disease severity) RG II and RG IV isolates were slightly virulent (mean of 4% disease severity), and RG III isolates were nonpathogenic (all 0% disease severity). Based on the ITS sequences, RG I and RG II are both D. bryonia, but based on RAPD fingerprints and pathogenicity, they represent two different genotypes and virulence groups.