No abstract

Diagnosis of yellow vine disease (YVD) in cucurbits, an important disease in the south-central United States, relies on external symptom appearance, phloem discoloration, and the presence of bacterium-like organisms (BLOs) in phloem. Polymerase chain reaction (PCR) amplification of BLO nucleotide sequences was explored as a means to improve diagnostic techniques. PCR, using a primer pair based on sequences of the citrus-greening BLO, amplified a 0.15-kilobase (kb) fragment from the DNA of symptomatic plants, but not from that of asymptomatic plants. Its nucleotide sequence suggested that the DNA amplified was of pro-karyotic origin. A primer pair, designed to amplify nonspecific prokaryotic 16S rDNA, amplified a 1.5-kb DNA fragment in both the symptomatic and asymptomatic plants. The 1.5-kb fragment from the asymptomatic plants corresponded to chloroplast 16S rDNA, and the band from the symptomatic plants was composed of 16S rDNAs from both chloroplasts and a prokaryote. The nucleotide sequence of the prokaryotic DNA was determined and used to design three primers (YV1, YV2, and YV3). Fragments of 0.64 and 1.43 kb were amplified with primers YV1-YV2 and primers YV1-YV3, respectively, from symptomatic plants. Neither primer set yielded fragments from asymptomatic plants, unrelated bacteria, or selected soilborne fungal pathogens of cucurbits. Phylogenetic analysis indicated that the prokaryote is a gamma-3 proteobacterium. The consistent association of the 0.64- and 1.43-kb fragments with symptomatic plants suggests that the gamma-3 proteobacterium may be the causal agent of YVD of cantaloupe, squash, and watermelon.

This study was conducted to better understand the nature of powdery mildew resistance in flowering dogwood (Cornus florida L.) and in particular to determine if inducible plant defense proteins are associated with powdery mildew resistance. Results from this study showed an accumulation of a new protein in resistant plants, but not in susceptible plants that were challenged with powdery mildew pathogen (Erysiphe (Sect. Microsphaera) pulchra). The protein accumulated in a high level in the resistant selections at 48 hr after inoculation with the pathogen and that was consistent with the production of pathogenesis related (PR) proteins. The protein was characterized as having an isoelectric point of 7.5 ± 0.5 and molecular weight of 18 ± 2 KD. Partial sequence analysis of this protein revealed homology with PR-10 protein associated with drought resistance in potato and was analogous to other proteins related to resistance in other crops. Repeated analysis showed similar results and suggested that a biochemical mode of resistance involving plant defense proteins may be associated with powdery mildew resistance in flowering dogwood.