Effect of phosphate and the arbuscular mycorrhizal fungus Glomus intraradices on disease severity of root rot of peas (Pisum sativum) caused by Aphanomyces euteiches

The effects of inorganic phosphate levels and the presence of arbuscular my corrhiza on disease severity of Aphanomyces euteiches in pea roots were stu died. Disease severity on roots and epicotyl as well as the oospore number within infected root tissue were correlated with the phosphorus (P) level i n the growth medium. The arbuscular mycorrhizal fungus Glomus intraradices increased P uptake and the P concentration in the plant but reduced disease development in peas. Polyacrylamide gel electrophoresis followed by densit ometry of glucose-6-phosphate dehydrogenase specific to A.euteiches was use d to measure the activity of the pathogen in roots. The enzyme activity inc reased with disease severity and disease incidence, except in plants supple mented with P at the highest level, where a peak in activity was seen 12 da ys after inoculation with the pathogen, followed by a decrease in activity. The epicotyl of mycorrhizal plants showed a reduction in disease severity although this part of the plants was not mycorrhizal. Thus, an induced syst emic factor may be responsible for increased resistance in mycorrhizal plan ts.