Late blight on potato in Flanders, Belgium: field trials and characteristics of the Phytophthora infestans population

Commercial fungicides were tested in the field for efficacy on foliar late blight caused by Phytophthora infestans. The fungicide treatments for late blight control were conducted at either 7- or 10-day intervals. The effect of the fungicide treatments on epidemic development, tuber rot and blight incidence and tuber yields were determined. Last summer late blight development was arrested in July and at the beginning of August due to high temperatures and lasting drought. Foliar disease severity significantly affected potato tuber yields. Lowest tuber yield was noted in plots with high late blight infection levels (nontreated control and an experimental mixture of organic acids (Vi-Care, 1 l/ha)) while highest yields were recorded in plots with low late blight infection. Late blight infection on leaf level was not significantly correlated with % tubers that showed late blight symptoms. No fungicide scheme completely arrested epidemic development under the environmental conditions of the trial. The effect of propamocarb hydrochloride + chlorothalonil (Tattoo C, 2.5 kg/ha) was less suppressive for P. infestans than the other fungicides tested for both interval systems. However, fenamidone + mancozeb (Sereno, 1.5 kg/ha), zoxamide + mancozeb (Unikat Pro, 1.8 kg/ha), dimethomorph + mancozeb (Acrobat, 2 kg/ha), cyazofamide + heptamethyltrisiloxane (Ranman 200 ml A/ha + 150 ml B/ha) and cymoxanil + famoxadone (Tanos, 0.6 kg/ha) controlled P. infestans most effectively for both interval systems. Also the other fungicides controlled foliar late blight sufficiently. Only small differences were observed between the different treatments. A total of 51 isolates of P. infestans were collected from disease outbreaks in commercial potato crops and private gardens in 2003. Isolates were recovered successfully from single lesions of diseased potato foliage. Not from all isolates pure cultures were obtained due to contaminations with Fusarium species and bacteria. The structure of the population was analysed phenotypically. Characteristics of the isolates included in vitro growth rate, mating type, in vitro sensitivity to the phenylamide fungicide metalaxyl-M and allozyme genotype at glucose-6-phosphate isomerase (Gpi) and peptidase (Pep) loci.