Studies on the assessment of yield losses caused by pre- and post-emergence damping-off and downy mildew of peas

Abstract

The effectiveness of systemic and non-systemic seed treatment fungicides and metalaxyl foliar applications on seedling establishment, root diseases and downy mildew of peas (Peronospora viciae) have been investigated in the field and under controlled conditions in Wisconsin tanks a glasshouse. The relationship between disease severity, cost of control and net profit have also been analysed.

The field trials were of two main types laid out in a completely randomised block design: 1). Two large trials sown at two sowing dates (21 September and 2 December 1983) in which there were five seed treatments:-

i. metalaxyl+captan (Apron SD 70 WP, 350g+350g a.i./kg). ii. metalaxyl (Apron SD 35 WP, 350g a.i./kg). iii. captan (Orthocide 65 WP 650g a.i./kg). iv. drazoxolon (Milcol 30% a.i./l) and v. an untreated control.

Three additional seed treatments the same as i. and iii. above, were given metalaxyl foliar sprays (Ridomil MZ 72 WP – metalaxyl 8% a.i./kg and mancozeb 64% a.i/kg).

2). Two subtrials within the main trials of five seed treatments each (as above) and also planted at two sowing dates (23 September and 7 November 1983).

The effectiveness of the fungicides used in the field trials were determined under controlled conditions in Wisconsin tanks at three soil temperatures 8, 12 and 16°C.

The trials showed that:

Seedling establishment was greatly enhanced by chemical seed treatment especially treatments with metalaxyl.

Seed treatment with metalaxyl not only increased plant density per unit area, but also reduced the severity of Peronospora viciae on seedlings for five weeks which enhanced growth and subsequently increased the dry pea yield.

Captan and drazoxolon conferred protection only to the seeds against damping-off, but had no effect on reducing downy mildew severity.

In all field trials the overall highest dry pea yields were obtained with metalaxyl treatments.

In Wisconsin tank trials, metalaxyl-treated seed or seeds sown in soil incorporated with metalaxyl granules, had a higher germination percentage. The effectiveness of metalaxyl in increasing seedling establishment was highest at 16°C.

Drazoxolon was very effective in increasing seedling establishment at 12°C.

Seedling death due to damping-off in untreated seeds, increased at higher soil temperatures.

Cost-benefit analysis indicated that metalaxyl seed treatment increased profit by a higher margin than all other fungicides. Cost-benefit ratios, while high for all fungicide treatments, were not good indicators of net profit.