Studies on the biology and control of certain foliar pathogens of lucerne (Medicago sativa)

Abstract

A review of the literature concerning foliar pathogens of lucerne and the methods used in controlling them has been made with particular reference to fungicides as a means of control.

The effect of a systemic fungicide, benomyl, and universal spraying oil on growth and germination of lucerne (Medicago sativa cv Wairau) was studied. Concentrations of 10,000 ppm benomyl and 5% oil were toxic to both lucerne germination and growth. Low concentrations of benomyl (100 ppm) and oil (0.5%) were not toxic as measured by fresh and dry weights or plant lengths. The addition of oil to benomyl did not increase toxicity.

The effect of benomyl on growth and germination of Pseudopeziza medicaginis, Phoma medicaginis and Stemphylium botryosum was studied in vitro. Benomyl almost completely inhibited spore germination of Phoma medicaginis at a concentration of 32 ppm and completely inhibited Pseudopeziza medicaginis at 512 ppm. However Stemphylium botryosum spore germination (95%) was not adversely affected by 2000 ppm. Conidia of S. botryosum were found to germinate in the presence of benomyl at 4000 ppm to produce swollen and shortened germ tubes. Mycelial growth of Ph. Medicaginis was completely inhibited at 10 ppm benomyl but mycelia growth of S. botryosum was still recorded at 1000 ppm benomyl.

Benomyl (1000 ppm ) and universal spraying oil (1%) as a treatment in glasshouse tests showed that benomyl was not as effective systemically as it was as a protectant or eradicant fungicide. Activity was recorded against Ps. medicaginis S. botryosum.

Three field trials were conducted in the 1972/73 season to test the effect of benomyl on: herbage yield, seed yield and seed quality. In two herbage trials plots of lucerne sprayed with, a weekly treatment and an early and late treatment of benomyl (1000 ppm) and oil (1%) yielded 16% and 11% more dry matter than did unsprayed plots (average of two trials). Ps. Medicaginis and Ph. Medicaginis were the major pathogens during the trials and their control reduced defoliation from 30% in untreated to an average of 10.5% in treated plots and increased leaf-stem ratios from 0.65 in untreated to 0.79 and 0.89 in plots sprayed with a weekly and early and late application ( averages of two trials).

In a seed yield trial, plots of lucerne were sprayed with a fortnightly treatment and an early late treatment of benomyl (1000 ppm) and universal spraying oil (1% ppm). These plots yielded 13.1% and 44.8% more seed than did unsprayed plots in the same trial defoliation was reduced from 63.9% to 11.8% in the early and late treatment. Dry matter was increased by 20.6% and 22.0% respectively. From treated seed harvested from this trial there was no effect on seed germination of on 1000 seed weights compared to untreated seed. However the incidence of Ph. Medicaginis on seed was reduced from 11.3% in untreated to 7.2% in fortnightly treatments.