Physical mode of action of fungicides and fungicide alternatives used against strawberry powdery mildew

Abstract: As production of strawberry has shifted towards growth in polyethylene tunnels during the recent two decades, the severity of epidemics of powdery mildew (Podosphaera aphanis) has also increased. Current powdery mildew control relies on intensive use of fungicides. The mechanisms by which fungicides interfere with vital physiological functions of fungi, and sites of action, have been described for many fungicides. Fungicides or fungicide alternatives differ in their efficacy against different stages in the pathogen life cycle. Some are only effective as protectants, some also as curatives, while others may have an additional eradicant activity. The physical mode of action of the fungicides or fungicide alternatives used against P. aphanis is unclear in many instances. We conducted laboratory experiments with detached leaflets of strawberry cv. Korona to describe the efficacy and physical mode of action of fungicides and fungicide alternatives applied before and after infection, and after disease had appeared, thus revealing preventive, curative or eradicative effects. Treatments were: (i) water treated control, (ii) kresoxim-methyl (Candit), (iii) penconazole (Topas 100 EC), (iv) sulphur (Thiovit Jet), (v) rape seed oil (Odelia), (vi) mineral oil (JMS stylet oil), and (vii) polyether modified trisiloxane (Silwet Gold). The recommended rates of the above products were applied as follows: (i) preventive, detached leaflets were sprayed with each compound and then inoculated with fresh powdery mildew conidia; (ii) curative, detached leaflets were inoculated and incubated for 48 hours followed by application of the different compounds; (iii) eradicative, leaflets were inoculated and incubated for one week followed by applications. There were significant differences among treatments on germination of conidia, sporulation, and disease severity when applied as preventive, curative, or eradicative. Conventional fungicidal spray programs for the control of powdery mildew are often applied late in the course of the epidemics, after powdery mildew has been observed on the plant. Our results indicate that some fungicides or fungicide alternatives have little or no effect on established infections and subsequent development of the pathogen. The knowledge on physical mode of action of fungicides or their alternatives as protectant, post-infection, or eradicative is useful for the effective management of the disease and for their most efficient use.