Can Spore Sampler Data Be Used to Predict Plasmopara viticola Infection in Vineyards?

Abstract

Grapevine downy mildew (DM) is caused by the dimorphic oomycete Plasmopara viticola , which incites epidemics through primary and secondary infection cycles that occur throughout the season. The secondary infection cycles are caused by the sporangia produced on DM lesions. The current research examined the relationship between numbers of airborne sporangia and DM development on grape leaves to determine whether spore sampler data can be useful to predict the potential for secondary infections of P. viticola . Three years (2015–2017) of spore sampler data confirmed that sporangia are a common component of the airborne microflora in a DM-infested vineyard and that their numbers depend on weather conditions. For a total of 108 days, leaf samples were collected from the vineyard at 2- to 3-day intervals and incubated under optimal conditions for P. viticola infection. The numbers of airborne sporangia sampled on 1 to 7 days before leaf sampling were significantly correlated with the numbers of DM lesions on the leaves. The best correlation (r=0.59), however, was found for the numbers of viable airborne sporangia (SPV), which were assessed by using equations driven by the vapour pressure deficit. In Bayesian and ROC curve analyses, SPV was found to be a good predictor of P. viticola infection of grape leaves, with AUROC=0.821 and false positive predictions mainly occurring at low SPV. A binary logistic regression showed that a threshold of 2.52 viable sporangia m -3 air day -1 enables a prediction of no infection with a posterior probability of 0.870, which was higher than the prior probability of 0.574. Numbers of viable sporangia in the vineyard air is therefore a useful predictor of infection and especially of no infection. The predictor missed some observed infections, but these infections were not severe (they accounted for only 31 of 374 DM lesions).

A Weather-Driven Model for Predicting Infections of Grapevines by Sporangia of Plasmopara viticola

“…Data were collected in the vineyard described by Brischetto et al (2020). In brief, the vineyard was located on the campus of Università Cattolica del Sacro Cuore, Piacenza (northern Italy, 45° 2'N, 9° 43'E).…”

“…To ensure the presence of P. viticola in the vineyard, six vine shoots regularly located along two adjacent vine rows were inoculated with a suspension of P. viticola sporangia on 14 May 2015, 19 May 2016, and 18 May 2017. The inoculation method was described by Brischetto et al (2020).…”

“…Contour plots showing the relationships between temperature and wetness duration or relative humidity and the following elements used in the model (see Figure 1): (A) sporulation rate, SPOR; (B) mortality rate of sporangia attached to sporangiophores on downy mildew (DM) lesions, MOR'; (C) mortality rate of sporangia detached from sporangiophores, MOR"; (D) minimal requirements for infection; and (E) infection rate, INF. Equations were obtained from the following sources: (A) the model of Lalancette et al (1988a) for describing the sporulation of P. viticola on grape leaves, in which the numbers of sporangia per cm 2 of leaf were rescaled by dividing by the number at 20°C and 24 h of a wet period; (B,C) the models relating temperature and relative humidity, expressed as vapour pressure deficit, as used by Brischetto et al (2020) for the sporangia attached to or detached from sporangiophores; (D) the model of Magarey et al (2005) with the following parameters estimated from data of Blaeser and Weltzien (1979) and Caffi et al (2016), with R 2 = 0.87 for estimated vs. observed data: shortest wetness duration at optimal temperature = 2 h; minimal, optimal, and maximal temperature for infection = 4.0, 21.0, and 30.2°C, respectively; and (E) the model of Caffi et al (2016) for infection of grape leaves by P. viticola.…”

“…the i th sampling day began at 10:00 h of the day i th-1 and ended at 09:00 h of the i th day. P. viticola sporangia m −3 of air on each sampling day (total number of sporangia, SPT, m −3 air day −1 ) were counted using a microscope (20x magnification; Brischetto et al, 2020).…”

“…The occurrence of infection was assessed by counting the DM lesions formed on grape leaves that were exposed to sporangia in the vineyard and that were then incubated in the laboratory under optimal conditions for infection, as explained in Brischetto et al (2020). In brief, at 2-or 3-day intervals at 10:00 AM, 20 random leaves without visible DM symptoms were collected in the two rows of vines where P. viticola had been inoculated and the spore sampler operated.…”