Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/10/2005
Publication Date: 4/1/2005
Citation: Deahl, K.L., Jones, R.W., Wanner, L.A., Plant, A. 2005. Phytophthora Infestans on Solanum Sarrachoides in Northeastern Maine. Plant Disease. 89:435. Interpretive Summary: Research was undertaken to find the cause of a new leaf blight disease on a common weed, hairy nightshade, a close relative to potatoes and tomatoes that was found in Maine in 2004. The pathogen was identified as the same organism that causes late blight in potatoes an tomatoes. The isolates from these nightshade plants caused the late blight disease on tomatoes and potatoes. 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 hairy nightshades 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 Maine, the plant pathogen, Phytophthora infestans, was isolated and characterized from hairy nightshade weeds in a potato fields naturally infected with the organism. Koch's postulates were completed for three isolates. The three 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 nightshade isolates to potato isolates from the same area. The three nightshade isolates were all of the A2 mating type and were resistant to the commonly used fungicide, methaxyl (Ridomil). Growers who cultivate more than one Solanaceous species (potato, tomato, petunia etc.) should be aware that hairy nightshade weeds may have Phytophthora infections that can serve as clandestine reservoirs of inoculum. Because these plants do not show conspicuous symptoms, they may escape proper detection and removal or treatment It is recommended that when Phytophthora is found in weeds growing within or around crops like potato, 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. hairy nightshade, as these common weeds may play a major role in the introduction and spread of damaging pathogens to new locations. These strains are aggressive pathogens on potato and tomato plants.