FAILURE IN THE CONTROL OF DOWNY MILDEW (PERONOSPORA PARASITICA) ON BRASSICA SEEDLINGS WITH PHENYLAMIDES IN ITALY
A BRUNELLI, P F'LORI and M COLLINA
Centro di Fitofarmacia, Universita di Bologna, Via Filippo Re 8, 40126 Bologna, Italy
Background and objectives
In the autumn 1997 two greenhouse trials were carried out in a big nursery in North-east Italy (Rovigo province) where metalaxyl treatments did not provide adequate control of the disease especially during the late summer growing cycles. The tests were made on seedlings of cauliflower (Brassica oleracea var. botrytis, cv Pamir), broccoli (B. o. var. italica, cv Maraton), and kohlrabi (B. o. var. gongylodes cv Quick Star) grown in 228-module polystyrene trays. Different fungicides at recommended rates were tested : metalaxyl. oxadixyl, cymoxanil and propamocarb (everyone in mixture with chlorothalonil) and chlorothalonil alone. Moreover metalaxyl was applied as seed treatment and by soil drench before emergence. Two or three treatments were applied respectively each 8 or 4 days starting from eight-ten days after sowing. Four replications (trays) were used for each cultivar in a randoniized blocks design. Water sporangial suspension of shredding fresh plant material collected from the nursery itself, was applied on cotyledons two days after the first fungicide application. Disease severity was assessed as percentage of leaf area infected for both cotyledons and primary leaves.
Sporulated leaves coming from untreated trays of the greenhouse trials, were used to obtain fresh inoculum for laboratory tests (adapted procedure described by Crute et al. .) Vermiculite was placed into closed glass-pots and satured with Hewitt-nutrient solution (40 ml/pot) amended with the appropriate amount of metalaxyl technical grade dissolved in methanol. Concentrations used ranged from 0.01 to 200 Rg a.i./ml in 5 fold steps and two glass-pots per concentration were prepared. Broccoli seeds (12-15) of the cultivar Maraton were sown onto the surface of the vern-dculite. The seedlings were allowed to grow for 7 days at 20'C (12 hrs photoperiod), to obtain fully expanded cotyledons. Water spores suspension (100.000 sporangia/ml) was sprayed onto cotyledons. Seven days after inoculation the percentage of cotyledons bearing spores was evaluated.
Results and conclusions