Pakistan grows a large variety of vegetables of tropical, sub-tropical and temperate groups on 150,000 hectares producing about 1.8 million tones. According to experts of human nutrition, a balanced diet requires 100 gms of vegetables per person per day. Vegetable crops could thus be used to supplement the cereal crops.
Botanically, vegetables can be classified into four categories:
Herbage; coriander, mint, spinach, lettuce, fenugreek.
Underground root: Beet, carrot, radish and turnip.
Underground stem: Potato, onion, garlic, ginger.
Fruit: Brinjal, tomato, chilli, lady's finger, and cucurbitaceous plants.
Seedborne diseases attacking vegetables often inflict severe losses under field conditions. They cause death and decay of seeds and seedlings before emergence. Others attack roots, stem, leaves or fruits. According to a field survey conducted during the growing seasons, 5-10 percent losses generally occur in vegetables in normal conditions.
To obtain maximum output of vegetables irrespective of areas whether bit or small, the primary need is to have the correct identification of the pathogens involved and their control. An account of symptoms, methods of detection and their control is described.
Symptoms: Leaf spot blight - It is an important common disease of vegetables and affects the yield potential. In Pakistan, the disease generally occurs in plains but it may also occur on hills. It becomes quite serious under warm and humid weather. The disease is caused by a fungus Alternaria solani which produces dark-brown concentric spots on leaves and their margin which curl necrotic and dry. With the severity of infection, tubers or potato plants become undersized. The affected tomatoes may begin rotting.
Tuber rot: A fungus Fusarium oxysporum causes Tuber rot. The incidence of tuber rot stored at high temperature is higher. The most prominent symptom is wet rot. The affected tubers show brown discoloration, accompanied by watery and bad smell. The tubers stored at comparatively lower temperature develop dry rot in which case sunken, shrivelled and wrinkled areas occur on the surface of the tubers. Later, whitish growth of the causal fungus appears on the diseased tubers.
Late blight: The disease Caused by the fungus, Phytophtora infestants, is confined to the hills and damages the crop quite seriously. It appears in the form of water-soaked light brown lesions on tips and margins of leaves. These patches increase and involved the whole surface and leaf curls. Under severe conditions, the disease extends to stalk which rots into a pulp. The fungus infects tubers from inside soil, causing dry rot and brown discoloration of tissues. They infect the plant too much and it dies.
Pod and leaf spot: A fungus Ascochyta pisi, which appears on leaves but also produces infection on the stem, causes it. Later, the number of spots increases and joins with each other covering large areas. In pods, the fungus may attack the developing grains, causing diseased spots. The affected plant portion withers and dries. The disease spreads rapidly in rainy season but is checked in dry weather. It is see as well as soil-borne.
Wilt of pea, tomato and cucurbit: It is one of the common diseases of vegetables and causes an appreciable loss in production. Fungal pathogens viz, Fusarium oxysporum, F. solani and F. moniliforme are involved in causing wilt in the early growth stage of plants. The disease appears in the seedling and adult plant stages. Its initial symptoms develop in the form of seedling blight and root rot. It the adult plants, the disease generally show when the leaves begin yellowing, dropping of petiole, and leaf shedding. Control measures: panting certified seed Certified seed must be used for planting and the seed is certified only after thorough field inspection and laboratory tests. Only the certified seed of reliable and authorised source should be used.
Seed treatment: Certified seed is usually given necessary treatment for the control of certain diseases. Seed treatment includes the use of fungicides. These are applied by different methods. The seed in small lots is treated in simple seed treaters. The appliance commonly available for this purpose is the rotary seed dresser in which the dry seed dressing is done. In seed processing plants, slurry treaters are used in which requisite quality of a fungicide in the form of slurry gets automatically mixed with the specified quantity of seed before the latter is bagged. Protective measures: In severe cases, the seed gets infected in the field itself before the harvest. In such cases, it is essential to spray fungicides or bactericide to protect it from infection.
Harvesting: harvesting should be done in case of true seeds when the moisture content is optimum. Cracking of the seed occurs if it is allowed to dry too much in the field itself. On the other hand, excessive moisture content might lead to the development of mouldy growth on seed in the storage. In case of root crops, care should be taken during harvesting so that no injury is caused to the propagative parts since this injury can provide entry points to various organisms.
Storage: Quality of seed during storage mainly depends on the storage conditions. Temperature and relative humidity of the storage will largely influence the invasion by various micro-organisms. Seed should not be allowed to get infested by insects or diseases causing pathogens.
Diseased seed: Restrictions should be imposed on the movement of seed carrying a particular pathsogen to areas where it has hitherto not been observed and all the planting material should be thoroughly checked before its distribution to farmers.