Infection of linseed by Alternaria linicola : Effects of inoculum density, temperature, leaf wetness and light regime

Controlled environment studies were conducted to determine the effects of inoculum density, temperature, leaf wetness and light regime on the infection of linseed by Alternaria linicola. The % cotyledons and leaves with symptoms, and the disease severity (% leaf area with symptoms) increased linearly when the inoculum density increased from 1 x 10(3) to 1 x 10(5) conidia ml(-1). The first symptoms appeared on cotyledons and leaves 4 and 6 days after inoculation, respectively. Eight hours of leaf wetness were sufficient to initiate the disease at 25 degrees C but not at 15 degrees C, when 10-h periods of leaf wetness were required. % leaf area with symptoms was lower at 15 degrees C than that at 25 degrees C irrespective of the leaf wetness periods tested. Interruption of a continuous leaf wetness period by a 12-h dry period, occurring at any time between 1 and 18 h after inoculation, decreased the % cotyledons with symptoms and the disease severity, with the greatest reductions (60% and 100%, respectively) being observed when the dry period began 6 h after inoculation. A. linicola conidia were able to exploit successive 12-h periods of leaf wetness cumulatively to infect linseed plants. Disease incidence and severity were positively correlated with the dark period following inoculation, but they were negatively related to the length of the initial light period. Our findings suggest that infection of linseed by A. linicola and further development of symptoms can occur under unfavourable environmental conditions.