5.5.32
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
Crucifer downy mildew (Peronospora parasitica Pers. ex Fr.) occurs frequently on brassica crops in North Italy, however the most damaging effects are restricted to young seedlings growing in nurseries within greenhouse. Control of the disease rest on scheduled applications of fungicides and, among others,, the phenylamides are widely and successfully used in mixture with protectants. Recently failure in control with phenylamides has been notified. Therefore to establish whether the reduced efficacy was caused by the possible resistance of P. parasitica to phenylamide chemicals, as already reported in other areas [1, 2], tests in greenhouse and in laboratory were carried out.

Materials and methods
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. [2].) 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
In greenhouse tests a satisfactory control of the disease was obtained with propamocarb and eymoxanil in mixture with chlorothalonil while metalaxyl and oxadixyl in mixture with chlorothalonil exerted only a scanty activity, not much higher than chlorothalonil applied alone. Insufficient effectiveness was also showed by metalaxyl applied both as seed dressing and soil drench. In sensitivity tests carried out in laboratory, the isolate grew and sporulated profusely on broccoli seedlings treated with metalaxyl at 100 and 200 micro g a.i/ml. This sensitivity level is equal to that found by Crute et al. [2] in resistant isolates of P. parasitica in UK. Taking into consideration that metalaxyl based fungicides were successfully used for many years in the nursery of the experimental trials, our laboratory results and the lack of activity showed by metalaxyl and oxadixyl in greenhouse tests can suggest the occurrence in Italy of resistance to phenylamide fungicides in crucifer downy mildew (P. parasitica).

References
1. Brophy TF and Laing D, 1992. Crop Protection 1 1, 160-64.
2. Crute IR, Norwood JM, Gordon PL, 1985. Fungicides for Crop Protection, BCPC Monograph No 31, p. 311-14.