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United States Department of Agriculture

Agricultural Research Service


item Pennypacker B W
item Knievel D P
item Risius M L
item Leath K T

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/10/1994
Publication Date: N/A
Citation: N/A

Interpretive Summary: Verticillium is a serious disease of alfalfa caused by the fungus Verticillium albo-atrum. In order to grow alfalfa profitably, alfalfa plants must be selected that are resistant to attack by this fungus. This research explored one of the ways by which plants are resistant. Resistant plants were found to continue to produce carbohydrates even when infected, but susceptible plants could not. The sustained production of carbohydrates is prerequisite to the initiation of subsequent resistant responses. Resistant plants behaved like susceptible plants at levels of light too low for adequate production of carbohydrates. This research helps us to understand how alfalfa and other plants resist wilt diseases.

Technical Abstract: Photosynthetic photon flux density (PPFD) was manipulated in greenhouse experiments to determine whether carbon assimilation regulated the expression of resistance to Verticillium albo-atrum. Treatments were pathogen (V. albo-atrum and no V. albo-atrum), clone (resistant and susceptible), PPFD (100, 70 and 40% of ambient), and time (3 wks). Treatment effects on disease ratings, dry weight of plant parts, net photosynthesis, and stomatal conductance were evaluated weekly. Significant pathogen x PPFD x week interactions were detected in disease rating, plant height, stem dry weight, and aerial biomass, and a pathogen x PPFD interaction was noted in leaf dry weight, when data from the resistant clone were analyzed. In all cases, the interactions were caused by the loss of resistance in resistant plants grown under 40% PPFD. V. albo-atrum did not affect net photosynthesis or stomatal conductance of the resistant clone, but these parameters were reduced by the 40 and 70% PPFD treatments. Carbon assimilation was critical for the expression of resistance. The susceptible clone failed to respond to PPFD levels when treated identically to the resistant clone, which did exhibit a differential response to PPFD. The inability of the susceptible clone to alter its response to V. albo-atrum is evidence that the defense mechanism under investigation is not simply a constitutive part of alfalfa, but is unique to the resistant clone.

Last Modified: 05/24/2017
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