Epidemiology of Alternaria linicola on linseed (Linum usitatissimum L.)
Conidia of A. linicola germinated over a wide range of temperatures (5 - 25°C) on both agar and leaves. Germination started within 2 h after inoculation at temperatures between 10°C and 25°C, either on agar or on leaves. At 5°C, there were lag periods of 2 and 4 h before the initiation of germination on agar and on leaves, respectively. Germinating A. linicola conidia were very sensitive to drying between 2 and 6 h after inoculation. In the presence of leaf wetness, light applied before the initiation of germination delayed the germination process and decreased the length of the germ tubes. Light applied after the onset of germination decreased both the percentage of conidia which germinated and the length of the germ tubes. In the absence of leaf wetness, light applied before or after the initiation of germination stopped the germination process or decreased the percentage of conidia which germinated, respectively. Conidia of A. Linicola germinated by producing germ tubes and occasionally by producing secondary conidia. Formation of appressoria was inhibited at 5°C. Penetration of the leaf tissues started 12 h after inoculation at 15°C and occurred mainly directly through the epidermal cells and occasionally through stomata. A. linicola is a "diurnal sporulator". In vitro most isolates sporulated only after exposure to diurnal NUV-light. However, for some isolates exposure to diurnal NUV-light did not seem to induce sporulation unless the mycelium was wounded and grown on a medium rich in CaCO3 (S-medium) at high relative humidity. In vivo sporulation of A. linicola was increased after induction by light. The greatest numbers of conidia were produced under continuous leaf wetness and alternating dark/light periods (12 h each). Under these conditions the number of conidia produced increased with increasing temperature from 10°C to 20°C. Alternating 15°C/10°C or 20°C/15°C day/night temperatures decreased the number of conidia produced compared with the constant temperatures 15°C and 20°C, respectively. In controlled environment studies, infection of linseed plants by A. Linicola and development of symptoms was affected by the leaf wetness period, its interaction with temperature and by the light conditions. Eight hours of leaf wetness were sufficient to initiate the disease at 25°C but not at 15°C when a longer period of 10 h was needed. Infection of linseed plants by A. Linicola occurred under interrupted leaf wetness periods at 15°C, but the incidence and severity of the disease was lower than that under continuous leaf wetness. The disease incidence on stems and the disease severity on leaves was negatively correlated with the length of the light period applied immediately after inoculation. Disease incidence and severity increased with increasing inoculum concentration from 1 x 10³ to 1 x 105 conidia ml-¹. The cotyledons appeared to be more susceptible to A. linicola infection than the leaves when the same inoculums density was used. A. linicola was detected on 12 of the 20 seed samples tested and on six of them at a high incidence (> 50%). Seed seems to be the main source of primary inoculum as the pathogen was effectively transmitted from infected seeds to the emerging seedlings. Infected linseed stem debris, volunteer linseed plants and the weed Veronica agrestis were also sources of primary inoculum for the infection of linseed crops by A. linicola. Structures resembling chlamydospores formed in the mycelium and conidia of A. linicola seem to be involved in the survival of the pathogen in stem debris. Conidia of A. linicola were mainly dispersed by the wind (air-borne conidia) and their dispersal followed seasonal and diurnal periodicities, which were influenced by the weather conditions and the incidence of the disease in the crop. The greatest numbers of A. linicola conidia were collected by the Burkard spore sampler on the first dry day following periods of rain, between 12:00 h and 13:00 h and during the period between flowering and harvest of the crop (July - September). Bait plants were more efficient than the Burkard spore sampler in detecting A. linicola conidia present in the crop early in the growing season. The number of A. linicola conidia dispersed within a linseed crop decreased with increasing height above ground, but some conidia were collected 80 cm above the crop canopy. The number of A. linicola conidia dispersed downwind from a line inoculum source decreased with increasing distance from the source and by the end of the growing season conidia were collected by up to 40 m from the source. When the A. linicola disease gradients were studied from point or line inoculum sources, the disease incidence decreased with increasing distance from the inoculum source. By the end of the growing season, the disease was detected 20 or 60 m from the point or line inoculum sources, respectively. Multiple applications of iprodione or prochloraz sprays to control A. Linicola infection in the crop, especially the seed-borne phase of the pathogen, and to increase crop yield gave variable results depending on the weather conditions and the incidence of the disease in the crop. Multiple applications of benomyl or chlorothalonil sprays had either no effect or increased the incidence of the disease in the crop.