Studies on aphanomyces root rot of peas (Pisum sativum) caused by Aphanomyces : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in the University of Canterbury [Lincoln College]

Abstract

This disease was recorded in 1978 in New Zealand, and found to be a serious disease of peas in the South Island. Disease avoidance is possible, i.e. growing peas only in infested soils with low or moderate amount of inoculum. The literature on the pathogen and factors affecting development of the disease are reviewed, and these aspects studied to determine alternative control methods.

When soil samples from 18 infested fields were tested by the Disease Severity Index method (DSI) and direct counting of oospores, there was a curvi-linear relationship (r = 0.97***) between DSI and number of oospores in soil. These results were confirmed when a known number of oospores produced in culture were added to non-infested soil.

On potato dextrose agar, growth of Aphanomyces euteiches was inhibited by water potentials between -20 and -30 bars depending on temperature. In glasshouse trials with infested soils, disease increased linearly between -200 and -3 millibars in pea plants grown at 8°, 12° , 16°, 20° and 25°. Yields decreased with increased moisture levels and temperatures.

The disease in peas was related to both the inoculum and growth stage at time of infection. Yield was reduced by 35% when the moisture of an infested soil was raised to field capacity (0 millibars) at 3 weeks after sowing, but yield was not affected in plants in soil raised to field capacity at the flowering or podding stages.

Aphanomyces euteiches parasitised Medicago sativa, Trifolium repens, Stellaria media, Viola arvensis and Capsella bursa-pastoris in the glasshouse and field. Infected plants showed no symptoms, but the fungus could be isolated from their roots, and each of the isolates infected peas which showed typical symptoms.

The effect of the fungicides, pyroxyfur, metalaxyl and hymexazol was assessed on Aphanomyces euteiches in vitro. Pyroxyfur at 1 ppm inhibited hyphal growth while significant reductions occurred at > 10 ppm with metalaxyl and hymexazol. Zoospore formation was inhibited by pyroxyfur at 10 ppm and by metalaxyl and hymexazol at > 100 ppm, whereas zoospore motility was inhibited by pyroxyfur and hymexazol at 1 ppm and metalaxyl at 5 ppm. Pyroxyfur and metalaxyl at 1 ppm significantly reduced oospore formation with hymexazol effective at 10 ppm.

The same fungicides were tested as seed treatments in glasshouse trials. After 4 weeks of growth all treatments showed similar levels of control. After 12 weeks growth, with soil of DSI 50, the disease was significantly reduced by metalaxyl (-26%) and hymexazol (-45%), but with soil of DSI 100, no differences were observed. In the same trials, fungicide combinations did not improve the efficacy of seed treatments. In a separate trial, pyroxyfur treated seed grown in soil with DSI 100 reduced DSI significantly (-18%) after 6 weeks with root dry matter being increased by 39% and total dry matter by 23%. Significant increases in top dry matter occurred at 2 weeks 0(+44%) and 8 weeks (+78%).

There were no significant interactions between the fungicide treatments, moisture levels (-200, -28 and -3 millibars) and temperatures (20° and 24°).

Growing Brassica napus in infested soils for 4 months reduced the DSI; from originally 58 to 27, from 72 to 53, and from 81 to 56. There was no change in DSI when Triticum aestivum, Trifolium repens or no plants were grown in the same soils. When dried leaves and stems of 5 brassica species were incorporated (0.5% of the soil dry weight) into infested soil, there were significant reductions in DSI and oospore numbers. The trend in reductions with length of time of incorporation was similar with greater effect from 6 weeks incorporation compared to 3 weeks. With soils of DSI 50, 75, and 100, the mean effect of Brassica napus, Raphano-brassica and Sinapis alba were equally effective, reducing DSI by 32-38% and oospore numbers by 66-72%.

When the same brassica species were grown for 6 months in infested soils and subsequently the roots alone incorporated into the soils for 6 weeks, there was an average reduction of 41% in DSI and 56% in oospore numbers by Brassica oleraceae, Brassica napus and Raphano-brassica. This cultural method, by reducing inoculum, is of value in the control of aphanomyces root rot.