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United States Department of Agriculture

Agricultural Research Service


item Deahl, Kenneth
item Fravel, Deborah

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2003
Publication Date: N/A
Citation: N/A

Interpretive Summary: Research was undertaken to find the cause of a new leaf blight disease on petunia that was found in Maryland in 2001 and 2002. The pathogen ,sporulating on the plants, was isolated in pure culture, inoculated onto new plants,and found to cause the same symptoms as the original disease. The pathogen was reisolated again, thus demonstrating that the fungal-like pathogen, Phytophthora infestans.Identification of P. infestans was based on the appearance of this organism in culture and microscopically. These same isolates were tested on other plants in the same family (Solanaceae) and caused disease on nightshades, tomato and potato (late blight). These isolates were found to be resistant to pesticides usually applied to control late blight on potato. This information will be used by extension agents and growers who should be aware that petunia can be a reservoir for the pathogen and that the pathogen should be tested to determine if it is resistant to pesticides before control measures are initiated.

Technical Abstract: For the first time in the state of Maryland, the plant pathogen, Phytophthora infestans, was isolated and characterized on greenhouse grown petunias. Koch's postulates were completed for three isolates. The three petunia isolates were also pathogenic on potatoes and tomatoes, which are historically the primary targets of this microbe. RFLP analysis demonstrated the close similarity of the petunia isolates to potato isolates. The three petunia isolates were all of the A2 mating type and were resistant to the commonly used fungicide Subdue (Ridomil). Therefore, greenhouse growers who cultivate more than one Solanaceous species (potato, tomato, nightshades etc.) should be aware that petunia transplants may have incipient Phytophthora infections that can serve as clandestine reservoirs of inoculum. Because these plants do not show conspicuous symptoms, they may escape proper fungicidal treatment. It is recommended that when Phytophthora is found in a greenhouse crop like petunia, the pathogen species should be identified and it should be tested for resistance to fungicides before disease control strategies are implemented. The report underscores the need for continued monitoring of the pathogen populations in bridging hosts, e.g. petunia, as the transportation of petunia transplants may play a major role in the introduction of damaging organism to new locations since these strains are aggressive pathogens on potato and tomato plants.

Last Modified: 06/21/2017
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