Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/1997
Publication Date: N/A
Interpretive Summary: Since 1991, a new disease of cucurbits, called yellow vine, has destroyed thousands of acres of cantaloupe, squash, pumpkin, and watermelon in Texas and Oklahoma costing millions of dollars. Much controversy has arisen due to misdiagnosis of the causative agent, ranging from seed-borne Fusarium wilt, herbicide drift, and a number of soil-borne fungi. A methodical approach was undertaken to determine the cause of cucurbit yellow vine. W made hundreds of isolations from yellow vine affected plants that failed to show a consistent association of any microorganism with the disease. Through a series of inoculations appropriate for an unknown disease, fungi, bacteria, and viruses were systematically eliminated. Because of the obvious phloem damage associated with yellow vine and the failure to consistently associate bacteria and fungi with the disease, efforts were directed to the possibility of an unusual type of bacterium that cannot be cultured. Light microscopy and scanning electron microscopy revealed that the phloem of affected plants was clogged and obstructed with an unknown substance. Using transmission electron microscopy, bacteriumlike organisms were observed in the phloem of yellow vine affected cantaloupe, squash, pumpkin, and watermelon. Studies have suggested that transmission is by insects but the vector is at present unknown. Additional efforts will be needed to characterize the bacterium and develop disease control strategies for the disease.
Technical Abstract: Since 1991, a new disease of cucurbits in central Texas and Oklahoma, designated yellow vine, has resulted in the decline and plant death of watermelon, cantaloupe, squash, and pumpkin. Affected plants are characterized by leaf yellowing, phloem discolorations, and plant collapse. Interannual disease incidence has ranged from spotty outbreaks to complete crop loss in early-planted watermelon fields. A systematic investigation to determine the causal agent of the disease included pathogen isolation attempts, transmission tests, serological assays with various antisera (ELISA and western blottting), and DNA hybridizations with selected probes (dot and Southern blots). None of these tests revealed a consistent relationship between the expression of yellow vine symptoms and the presence of a particular microorganism or virus in the plant. However, transmission electron microscopic examination showed the consistent presence of a bacteriumlike organism (BLO) in the phloem sieve elements of symptomatic plants. The rod-shaped BLOs observed only in symptomatic cucurbits, measured 0.03 um in width and 1.0 to 2.0 um in length and were surrounded by a triple layered cell envelope.