A simple model describes development of early peaks in oomycete zoospore inoculum detected in southern UK outdoors horticultural reservoirs

Abstract

The numbers of water-borne oomycete propagules in outdoor reservoirs used in horticultural nurseries within the UK are investigated in this study. Water samples were recovered from 11 different horticultural nurseries in the southern UK during Jan-May in 2 "cool" years (2010 and 2013; winter temperatures 2.0 and 0.4°C below UK Met Office 30 year winter average, respectively) and 2 "warm" years (2008 and 2012; winter temperatures 1.2 and 0.9°C above UK Met Office 30 year winter average, respectively). Samples were analyzed for total number of oomycete colony forming units (CFU), predominantly members of the families Saprolegniaceae and Pythiaceae, and these were combined to give monthly mean counts. The numbers of CFU were investigated with respect to prevailing climate in the region: mean monthly air temperatures calculated by using daily observations from the nearest climatological station. The investigations show that the number of CFU during spring can be explained by a linear first-order equation and a statistically significant r 2 value of 0.66 with the simple relationship:where a is the rate of inoculum development with temperature T, and b is the baseload population at temperatures below T b . Despite the majority of oomycete CFU detected being non-phytopathogenic members of the Saprolegniaceae, total oomycete CFU counts are still of considerable value as indicators of irrigation water treatment efficacy and cleanliness of storage tanks. The presence/absence of Pythium spp. was also determined for all samples tested, and Pythium CFU were found to be present in the majority, the exceptions all being particularly cold months (January and February 2010, and January 2008). A simple scenario study (+2 deg C) suggests that abundance of water-borne oomycetes during spring could be affected by increased temperatures due to climate change.

Monitoring oomycetes in water: combinations of methodologies used to answer key monitoring questions

“…These inoculum surges are often initiated by changes in the environment such as increased irrigation frequency or by rainfall (Ristaino, 1991;Cafe-Filho et al, 1995), but can also result from subtle changes in cultural practice, for example sudden reductions in the root zone temperature in hydroponics crops (Kennedy and Pegg, 1990). There are large seasonal variations in the numbers of oomycete propagules seen in irrigation water in the UK (Pettitt et al, 2015), some likely driven by seasonal changes in water temperature (Pettitt and Skjøth, 2016). A fairly consistent double peak in general oomycete CFU counts is seen in clinic samples from UK nurseries, with larger counts in late Spring and in late Summer-Early Autumn and a decline in mid-Summer (Pettitt et al, 2015).…”