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CROP DISEASES moisture and making harvest very difficult. When the flowers are infected, you see small, pinpoint lesions that cause them to fall off and affect seed set. Infected pods may affect the size and quality of the seed. Infected seed will be small with a dark brown discoloura on. Disease Cycle Ascochyta blight survives in the soil, on the seed coat, in the seed, and on the pea straw residue. This disease is spread by two types of spores, one type by wind and the other type through rain splash. Ini al infec on usually occurs during wet weather, when wind-borne spores are carried from infected fields into new fields. In this way, the pathogen can move across large areas, including throughout an en re crop and even to nearby fields in which field pea may not have ever been grown. Later infec ons occur when the pathogen produces the other type of spores that are spread by rain splash to other leaves and nearby plants. A large number of spores can be spread by rain splash from an infected plant, although most of these spores are deposited nearby. The most cri cal factor affec ng the disease spread is leaf wetness or the level of moisture in the canopy. Canopy structure, lodging, previous injury, and plant density can all have an effect on leaf wetness and will impact the severity of the disease. UNDERSTANDING ASCOCHYTA/ MYCOSPHAERELLA BLIGHT Ascochyta blight is the most destruc ve foliar disease of pea in western Canada and around the world. It is caused by a complex involving three fungal pathogens: Ascochyta pisi, Mycosphaerella pinodes and Phoma pinodella. Of the three, M. pinodes is by far the most abundant, causing 90% of Ascochyta blight infec ons. It is found in all commercial pea fields. It interferes with photosynthesis, causes the crop to lodge and reduces the weight, number and quality of the seed. All field pea cul vars are suscep ble. Symptoms Symptoms appear within two to four days a er infec on under op mal condi ons and lesions appear on all above-ground por ons of the plant, including leaves, stems, flowers and pods. Symptoms of infec on are always more severe on the lowest parts of the plant. Infec on begins with many small purple spots on the leaves. Under drier condi ons these spots remain about the same, while under moist condi ons they get larger, and turn a brown to black colour. Spots on the stems are similar in colour and are usually found close to where the leaf a aches. These stem lesions slowly get larger and join together to completely girdle the stem and give the en re lower plant a purplish-black appearance. It is the breakdown of the stems that causes the crop to lodge and makes the problem worse by encouraging Management Management of Ascochyta is best achieved by first reducing the amount of available inoculum and then suppressing the subsequent infec ons. Pea debris must be destroyed if possible, clean seed must be used, and soil-borne inoculum must be managed. Careful selec on of cul vars and applica on of foliar fungicides are used to suppress further infec on. Ideally, cul var resistance would be the best long-term strategy, but breeding a empts to achieve this have been unsuccessful so far. The current most u lized strategy for management of this disease is the applica on of fungicides. A combina on of the above strategies would offer the best result, but the combina on would be determined by economics and the available op ons at the me. Problems will arise with some of these management strategies, as in a no- ll or reduced ll cropping system, where burial of the residue is not compa ble with the farm prac ces, or in areas where crop residue breakdown is slow. In some areas where the wind-borne spores are the major source of the infec on, crop rota on would be less effec ve, and fungicide applica on would have to be considered as the most effec ve op on. With fungicide applica on, the ming is cri cal. You need to consider that the fungicides are protectants and By Robyne Bowness Davidson, Pulse Research Scien st, Alberta Agriculture and Forestry 1 6

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