Rain-driven epidemics ofPhytophthora porri on leek

Abstract

White tip disease of leek (Allium porrum), caused by Phytophthora porri, was studied in field experiments. On fields infested by soil-borne inoculum (oospores), relatively short periods of explosive disease increase alternated with periods in which apparently no new infections occurred. The analysis of rain data and disease data, using a degree-day model for incubation periods at constant temperatures, confirmed the hypothesis that disease increase ofP. porri is significantly correlated with rain; Rid j was 0.91, 0. 41 and 0.51 in 1992, 1993 and 1994, respectively. Correlations were highest early in the season. Lack of correlation later in the season may be ascribed to the effect of lesion death, which may be caused by total or partial leaf death, by desiccation or by other fungi overgrowing P.porri, and to the effect of secondary infection by zoosporangia, which appears to be not so strongly rain-driven as primary infection. Zoosporangia were observed in fields on water-logged light-green lesions. High lesion densities of leaf tips and leaf units at 10-20 cm above the leaf axils indicated that most infections depend on free water, either in puddles or in a water basin near the leaf axils. Although disease correlates well with rain data, disease forecasts will be unreliable as long as rain forecasts are unreliable.

New Insights in the Life Cycle and Epidemics of Phytophthora porri on Leek

“…When splash dispersal was suppressed by growing leek under a hoop greenhouse, P. porri was not able to initiate disease, except at the borders of the open hoop house. Moreover, in open field, disease increase in the observation period is correlated with the average daily rainfall in the earlier infection period, confirming observations made by Smilde et al. (1996a).…”

“…In the 10 middle rows, 10 sequential plants were chosen at random, marked and monitored twice a week for the typical white lesions of P. porri as described above. The average daily increase in diseased plants in each observation period (usually 3 or 4 days) was calculated (expressed as % increase in diseased plants/day) and correlated with the average daily rainfall in an infection period, one incubation period before the observed interval of disease increase as described by Smilde et al. (1996a).…”

“…Leek cultivar Kenton (Nunhems seeds) was planted in 12 single row plots of 200 plants in 2006, 2007 and 2008. In the 10 middle rows, 10 sequential plants were chosen at random, marked and monitored twice a week for the typical white lesions of P. porri as described above. The average daily increase in diseased plants in each observation period (usually 3 or 4 days) was calculated (expressed as % increase in diseased plants ⁄ day) and correlated with the average daily rainfall in an infection period, one incubation period before the observed interval of disease increase as described by Smilde et al (1996a). At each time of disease observation, the shortest and longest incubation time were calculated as 92 and 154 degree-days above )3°C, respectively (based on the degree-day model determined by Smilde et al 1996b).…”

“…The pathogen readily produces oospores in diseased leaf tissue that enter the soil when leaves decay. The thick‐walled oospores survive the crop‐free period, although it is not known for how long, and constitute the primary inoculum to initiate disease (Smilde and van Nes 1992; Smilde et al. 1996a,b).…”

“…They established that, on average, a lesion appears at 120 degree‐days after infection. In field experiments, disease increase was significantly correlated with average daily rainfall in the infection period (Smilde et al. 1996a).…”