Crop Diseases PDF

CROP DISEASES Plant disease is an abnormal condition in the plant caused by an internal disorder (nutrition and stress) or infection (pathogens). OR Plant disease is a condition of plant deterioration and damage caused by various abiotic (non living) a...

CROP DISEASES Plant disease is an abnormal condition in the plant caused by an internal disorder (nutrition and stress) or infection (pathogens). OR Plant disease is a condition of plant deterioration and damage caused by various abiotic (non living) and biotic factors (living). Effects of diseases on crops ❖ Reduced Yields: Diseased plants often produce fewer fruits, grains, or vegetables, leading to lower overall yields. ❖ Crop Losses: Severe infections can result in complete crop loss ❖ Stunted growth- when diseases attack a plant, it will not reach its full size. ❖ Discolouration- When a plant is attacked by disease, the leaves and stems may become yellow, reducing photosynthesis rate leading to poor growth. Classification of crop diseases Causative Agent Disease group Examples Bacteria Bacterial Diseases Bacterial wilt of Tomatoes & Potatoes, Bacterial blight of cotton & cowpeas, Bacterial blight of Soya bean Fungi Fungal Diseases Damping-off of seedlings, Downey mildew of peas, Sorghum & Maize smut, Powdery mildew of beans, Early & late leaf spot of groundnut Viruses Viral Diseases Maize streak virus, Cowpea aphid born mosaic virus, Cassava mosaic virus, Groundnut rosette virus 9 1. Maize Streak Virus Pathogen: Virus Mode of infection: The virus is introduced into the plant tissue of the host plant by sap- sucking leafhoppers. Leafhoppers acquire the virus when feeding on infected plants and then feed on healthy ones thus transmitting the virus to them. Once in the plant, the virus multiples and moves in the sap and reaches plant leaves causing characteristic streaking. Harmful effects of maize streak virus on crops ❖ Leaves turn light green with yellow stripes running parallel to the leaf veins, reducing the photosynthetic area, growth, and yield of plants. ❖ Cobs are small and partially filled leading to reduced crop yields. ❖ Inflorescence may become sterile leading to reduced yield. Control methods of maize streak virus Cultural control: ❖ Use resistant varieties of maize available ❖ Remove infected plants and burn them. ❖ Early planting Chemical control of maize streak virus ❖ Spray leafhoppers with dimethoate to kill them 2. Maize Smut Pathogen: Fungus Mode of infection: Fungal spores remain in the soil for several years, these spores spread to young maize plants through wind, soil dust, and rain splashes. Fungal spores will germinate in moist air or in irrigation or rainwater that collects on the plant leaves, ears, tassels and other structures of the maize plant. Spores enter plants through wounds/cuts caused by insects, animals, and poor cultivation practices. This leads to infections that are visible within about 10 days. Harmful Effects of Maize Smut ❖ Large blackish lumps appear on the stems leaves and cobs. These break open to release a fine black powder (spores that spread diseases to other plants). ❖ Spores end up covering the whole grain, leading to a complete loss of Grains/cobs. 10 Control methods of Maize Smut ❖ Avoid injury to roots, stalks, and leaves when cultivating, spraying, and fertilizing the fields. ❖ Pull up and burn infected plants ❖ Plant-certified seed, if using own seeds collect them only from disease-free plants ❖ Practice crop rotation 3. BACTERIAL WILT OF TOMATOES Pathogen: Bacteria Mode of Infection: The pathogen is soil-borne and enters through root wounds. It Can also spread through splash irrigation water, contaminated planting materials, and soil or implements and feet of workers. Harmful Effects of Bacterial Wilt Of Tomatoes ❖ Block the vascular bundles especially the xylem thus interrupting water uptake hence temporary or permanent wilting of affected plants. ❖ Death of infected plants. ❖ Yellowing of leaves. ❖ Discolouration of the xylem vessels. ❖ Decay of the pith which may cause extensive hollowing of the stem. Control and Prevention of bacterial wilt of tomatoes Cultural Control Methods ❖ Use certified seeds and seedlings from the disease. ❖ Avoid using contaminated land. ❖ Leave planting land fallow for several seasons. ❖ Crop rotation between non-susceptible and susceptible crops. Chemical Control methods ❖ Spray infected plants with antibiotics (streptomycin, ampicillin, tetracycline and penicillin) ❖ Fumigate soil using chloropicrin or application of phosphorous acid to suffocate the Pathogen. Common Weeds Of Botswana ❖ Black jack (Bidens pilosa) ❖ Mexican poppy (Argenome mexicana) ❖ Thorn apple (Datura stramonium) ❖ Couch grass (Cynodon dactylon) 11