Influence of inoculum concentration, leaf age, temperature, and duration of leaf wetness on Septoria blight of parsley

The effect of inoculum concentration, leaf age, temperature, leaf wetness duration on Septoria petroselini infection and the development of Septoria blight of parsley were studied in controlled environment conditions. The mean number of lesions cm−2 of brush-inoculated leaf surface increased 72.4% with increasing inoculum concentration from 104 to 2×106 conidia ml−1. As leaves of parsley plants matured, they became more susceptible to S. petroselini infection. A nonlinear relationship was found between inoculum concentration and lesion area for young and older leaves. An inoculum concentration of 104 conidia ml−1 of S. petroselini seems to be required for disease development. Parsley plants were sprayed, inoculated with conidial suspensions of S. petroselini, exposed to leaf wetness durations of 0, 12, 24, 48, 60, and 72 h, then blight was allowed to develop for 3 weeks at 15, 17, 20, 23, 27, and 32 °C. Septoria blight developed at all temperatures tested. The mean lesion number increased in parsley plants as wetness duration increased at 20 and 23 °C, whereas disease development decreased in parsley leaves as wetness duration increased at temperatures of 15, 17, 27, and 32 °C. The optimum temperatures for lesion development were 20 and 23 °C. At those temperatures, the optimum leaf wetness duration was 72 h. Leaf wetness duration did not contribute to an increase in disease severity at 15, 17, 27, and 32 °C. Leaf area covered by pycnidia apparently increased in treatments at temperatures between 17 and 23 °C during the wetness period of 48 h. Low levels of Septoria blight on parsley develop across a wide temperature range. Leaf wetness duration significantly increases disease severity, but only at optimal temperatures.