Scottish Agricultural College, Aberdeen, UK

Background and objectives
Decision models for disease control use epidemiological data on plant pathogens to predict when infection occurs and how epidemics develop. For Pyrenophora teres there is a poor understanding of how metereological parameters, the host and inoculum interact. Accurate timing of fungicide is crucial in the control of P. teres because of a short latent period and the potential for rapid disease development during optimum environmental conditions.

The objectives of this research were to identify biological and meteorological parameters of epidemic development of P. teres in order to develop a working epidemiological model for use in a computerized decision support system.

Materials and methods
Controlled environment experiments were designed to assess the influence of temperature, relative humidity, photoperiods and varietal resistance on infection by P. teres and disease progression. Winter barley seedlings were inoculated with ascospores or conidia and incubated for photoperiods of 16, 12 or 6 h in a 24-h period with a range of relative humidities between 65 and 95%. Days to visible symptoms were recorded and per cent mean diseased leaf area per plant (DLAP) was assessed 7 days after first symptom expression. Two winter barley varieties were used to compare the influence of varietal resistance on latent periods: Lark, which is susceptible to P. teres, and Manitou, a moderately resistant variety.

Using the data from the controlled environment experiments, hypotheses were established to predict when infection would occur and when fungicide should be applied.The hypotheses were tested in field trials.

Results and conclusions
Temperature was found to be a key factor in infection by P. teres. At maximum temperatures below 10C disease symptoms did not develop. Infection was observed in seedlings inoculated with ascospores after 5 days at 10C but seedlings inoculated with conidia failed to develop disease symptoms. When maximum temperatures were raised to 12C, conidial infection was also observed. The effects were similar in both varieties [1].

At a constant temperature of 12C the length of the photoperiod had a significant effect on infection following inoculation with conidia. The greatest DLAP and shortest latent period was recorded when seedlings were incubated for a 16-h photoperiod with a relative humidity of 85-95%. Shorter light periods of 6 h resulted in longer latent periods and significantly lower DLAP compared with 12 and 16 h light and infection only occurred at 85-95% relative humidity.

Increases in relative humidity and photoperiods of 12 and 16 h had less effect on DLAP following inoculation with ascospores although latent periods were shorter at the maximum %r.h. however, in contrast to conidial infection, the shortest photoperiod resulted in the greatest DLAP.

Yield from plots treated with timed fungicide application according to the hypotheses were not significantly different from plots treated according to a standard fungicide programme. However, the fungicide dose was 50% greater in the standard programme.

The results were used to produce a working epidemiological model for disease management of P. teres using timed fungicide applications.

1. Cooper J, Wale SW, 1997. Proceedings 15th Long Ashton International Symposium, p.42.