By C. M. Ocamb
Cause The fungi, Alternaria brassicae, A. brassicicola, and A. japonica, can infect leaves, petioles, stems, flower parts, and seed pods of a wide range of Brassicaceae crops and weeds. These fungi survive on residues of infected crucifer crops and weeds, producing asexual spores (conidia) when conditions are conducive and the debris is on the soil surface. The conidia can be moved by wind and water. This is a frequent problem in the Willamette Valley of Oregon on other brassicas. The disease also is known as gray leaf spot, pod drop, and dark leaf spot.
Conidia produced on infected plant debris are considered the primary inoculum for disease outbreaks. Studies have shown that A. brassicae and A. brassicicola on infected plant debris can produce conidia for as long as the debris remains intact, with infected stem tissues releasing spores up to 23 weeks later and which can spread diseases across a field or to neighboring fields. Conidia are generally present throughout the cropping cycle, on dead or living host material, so the threat of disease build-up during conducive environmental conditions is fairly constant. Conidia produced by A. brassicicola infect plants through direct penetration and rarely infect though stomata. Wet weather when crops are mature can lead to a rapid build-up in disease levels but conidia are dispersed during warm, dry periods following wet conditions. Temperature and relative humidity can both modulate disease development but the genetics of the plant host as well as the respective Alternaria species appear to play a role in maximum and minimum temperature range that promotes disease development. Studies have shown that sporulation by A. brassicae and A. brassicicola requires a relative humidity level at or above 91.5% and 87% while optimum temperatures were reported to be 64 to 75ºF and 68 to 86ºF, respectively. In a separate study, growth and sporulation by A. brassicae occurred when relative humidity was above 50% and temperature was between 41ºF and 86ºF, but growth and sporulation increased as relative humidity increased and 73ºF was found to be the optimum temperature. A high relative humidity (>95%) or free moisture are required for spore germination. It has been reported that a minimum wetness period of four hours was necessary for infection of oilseed rape at 64ºF, and that disease severity increased with an increasing wetness period up to 12 hours. Germination of spores produced by A. brassicicola and subsequent development of disease occurs when ambient temperature is between 82ºF and 88ºF. Generally, A. brassicae can infect under cooler temperatures and more quickly than can A. brassicicola.
Alternaria brassicae and A. brassicicola can also be seedborne. Most of the seedborne presence is superficial, but studies have found A. brassicae and A. brassicicola occurring internally in seed. Alternaria brassicae gradually dies out during storage at room temperature, with Indian mustard seed being free of the pathogen after five months in storage. But internal infections by A. brassicicola may persist for much longer, up to 12 years in cabbage seed studies.
Symptoms Small, dark or yellow leaf spots first develop, enlarging to circular areas that are brown to gray in color with or without concentric rings and possibly with black or purple borders and/or surrounded by yellow halos. Sometimes the leaf spots are limited by leaf veins so the spots are angular in appearance rather than circular. The centers of leaf spots may be coated with sooty black spore masses, and can crack or may drop out, producing shot holes. Leaf spot size ranges from areas barely visible (0.1 inch or smaller diameter) to more than 2 inches in diameter. Leaf lesions can lead to premature defoliation. Stem and petiole lesions are elongated and dark brown to black in color. On infected seed pods, violet to tan to black spots develop, and large areas of the pods may be dark in coloration, and can lead to infected seeds that are smaller in size and shriveled in appearance. Pods with infected pedicels fail to develop and drop off the plant. Severely spotted pods dry, shrink, and may split open prematurely, allowing shrunken seeds to drop to the ground. Planting of infested seed can lead to seedling death when soil temperatures are warm.
Cultural control
- Rotate out of crucifers for at least three years. Chose varieties with resistance to Alternaria if available. Avoid planting spring and fall-sown crops in close vicinity to each other. If possible, time planting so that crops mature before conditions are favorable for disease.
- Plant clean, pathogen-free seed.
- Plow under infected plant debris.
Chemical control Protective fungicides are registered for canola and should be applied at the initiation of flowering, prior to disease development.
- Elatus (Group 11 + 7) at 7.3 oz/A at the end of flowering/early pod set. Make no more than one Elatus application per year. Preharvest interval is 30 days. 12-hr reentry.
- Miravis Neo (Group 7 + 3 + 11) at 13.7 fl oz/A at the end of the flowering/early pod set. Preharvest interval is 30 days. 12-hr reentry.
- Quadris Flowable (Group 11) at 6 to 15.5 fl oz/A. See label for application directions and resistance management. Preharvest interval is 30 days. 4-hr reentry.
- Tilt (Group 3) at 2.6 to 4 fl oz/A making an application at the end of flowering/early pod set. Make no more than one (1) application per season. Do not apply within 30 days of harvest. 12-hr reentry.
- Vertisan (Group 7) at 14 to 20 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action (non-Group 7). Preharvest interval is 21 days. 12-hr reentry.
- Premixes of Group 7 + 11 fungicides are available for use. Do not make more than two (2) applications per season.
- Priaxor Xemium Brand at 4 to 8 fl oz/A. Preharvest interval is 21 days. 12-hr reentry.