Effects of leaf wetness duration and temperature on infection of Prunus by Xanthomonas arboricola pv. pruni

Abstract

Xanthomonas arboricola pv. pruni is the causal agent of bacterial spot disease of stone fruits and almond. The bacterium is distributed throughout the major stone-fruit-producing areas of the World and is considered a quarantine organism in the European Union according to the Council Directive 2000/29/EC, and by the European and Mediterranean Plant Protection Organization. The effect of leaf wetness duration and temperature on infection of Prunus by X. arboricola pv. pruni was determined in controlled environment experiments. Potted plants of the peach-almond hybrid GF-677 were inoculated with bacterial suspensions and exposed to combinations of six leaf wetness durations (from 0 to 24 h) and seven fixed temperatures (from 5 to 35°C) during the infection period. Then, plants were transferred to a biosafety greenhouse, removed from bags, and incubated at optimal conditions for disease development. Although leaf wetness was required for infection of Prunus by X. arboricola pv. pruni, temperature had a greater effect than leaf wetness duration on disease severity. The combined effect of wetness duration and temperature on disease severity was quantified using a modification of the Weibull equation proposed by Duthie. The reduced-form of Duthie's model obtained by nonlinear regression analysis fitted well to data (R = 0.87 and R2adj = 0.85), and all parameters were significantly different from 0. The estimated optimal temperature for infection by X. arboricola pv. pruni was 28.9°C. Wetness periods longer than 10 h at temperatures close to 20°C, or 5 h at temperatures between 25 and 35°C were necessary to cause high disease severity. The predictive capacity of the model was evaluated using an additional set of data obtained from new wetness duration-temperature combinations. In 92% of the events the observed severity agreed with the predicted level of infection risk. The risk chart derived from the reduced form of Duthie's model can be used to estimate the potential risk for infection of Prunus by X. arboricola pv. pruni based on observed or forecasted temperature and wetness duration.

Fig 1. Effect of temperature (A) and leaf wetness duration (B) on relative disease severity caused by Xanthomonas arboricola pv. pruni on peach-almond hybrid GF-677 plants 21 days after inoculation.

Symbols represent the mean relative disease severity of two experiments and five plants per experiment. Error bars in the upper left (A) or right (B) corner represent the mean standard error.

Fig 2. Three-dimensional response (A) and contour plot (B) of the combined effects of temperature and leaf wetness duration on relative disease severity caused by Xanthomonas arboricola pv. pruni on peach-almond hybrid GF-677 plants.

The predicted values were calculated using the reduced form of Duthie’s model, corresponding to Eq 5. (A) The black points correspond to mean relative disease severity of two independent experiments and five plants per experiment observed 21 days after bacterial inoculation. (B) An area represents relative disease severity values lower than or equal to the label on the contour line on the right of the area.

Fig 3. Observed relative disease severity versus estimated values with the reduced form of Duthie’s model, Eq 5.

The regression line (solid) is slightly different from the dashed line, which corresponds to a fitted line with an intercept of 0 and a slope of 1.

Fig 4. Evaluation of the prediction capacity of the infection model for Xanthomonas arboricola pv. pruni describing the combined effects of leaf wetness and temperature under greenhouse conditions.

Comparison between observed disease severity (S, 0–100 range) versus predicted relative disease severity (S’, 0–1 range) by the reduced form of Duthie’s model, Eq 5, on GF-677 peach-almond hybrid plants 21 days after inoculation.