Epidemiology of Pyrenophora teres and its effect on grain yield of Hordeum vulgare
van den Berg, Cornelius Gerardus Jacobus
Recent surveys in Saskatchewan have shown that the prevalence of Pyrenophora teres Drechs., the causal agent of net blotch of barley (Hordeum vulgare L.), has increased. Severe epidemics have been observed in several commercial fields. The major objectives of this study were the investigation of the epidemiology of net blotch and the determination of the effect of net blotch on barley production. Conidium germination and time to infection were compared for two spot-type isolates (P. teres f. maculata) and one net-type isolate (P. teres f. teres) in a growth cabinet. Spot-type isolates germinated sooner than the net-type isolate between 10 and 20 °c. Time to infection was shorter for the spot-type isolates than for the net-type isolate at all temperatures. The relationships among local weather conditions, incidence of airborne conidia, barley development and sporulation were investigated in field experiments with the susceptible cultivar Elrose at Shellbrook, Saskatchewan in 1986 and 1987. Airborne conidia were observed daily throughout the growing season. Their incidence showed a diurnal pattern with a peak between 12:00 and 16:00 h. At night, frequent dew formation favoured infection. Infection of lower leaves was due to infection by primary inoculum produced on crop debris. Infection of upper leaves was due to primary inoculum and to secondary inoculum produced on the lower leaves. The effect of net blotch on barley production was investigated in an experiment using Tilt (propiconazole) on Elrose in 1985 and 1986 and on the moderately resistant cultivar Argyle in 1986 at Saskatoon, Shellbrook and Medstead, Saskatchewan. Repeated application of Tilt reduced the rate of disease progress on Elrose, but not on Argyle, where resistance reduced the rate of disease progress. Resistance slowed disease progress more than repeated application of Tilt. On Elrose, Tilt increased grain yield and kernel weight, but had no effect on number of tillers and number of kernels per spike. On Argyle, Tilt had no effect on any trait. The potential yield loss caused by net blotch was estimated at 50%. Infection of net blotch lowered grain grade through kernel discolouration and low test weight. Using the components of variance and covariance, strong correlations were obtained between treatment effects of grain yield and disease severity. However, no satisfactory critical point mode·l could be developed to predict percentage yield loss from disease severity. In Saskatchewan, infected crop debris is the most important source of primary inoculum of P. teres. Under favourable conditions with sufficient primary inoculum, severe epidemics can be expected on susceptible cultivars. Foliar fungicide application is not cost effective at current grain prices, therefore extended rotations or the use of resistant cultivars are the only practical means to control net blotch in barley.