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PESTS AND DISEASES OF CASSAVA PRESENTED TO THE RIO CLARO FARMER’S ASSOCIATION . ANTHONY SEESAHAI Consultant to the CFC/CARDI ROOT CROP PROJECT. PESTS AND DISEASES PESTS DISEASES 1. 2. 3. 4. 5. 6. 7. 8. 9. 1. 2. 3. 4. 5. 6. 7. 8. Thrips Chinch Bug Cassava shoot fly Mites Cassava hornworm Whiteflies Mealy bugs Grasshoppers Termites Cassava bacterial blight Superelongation African mosaic disease Bacterial stem rot Tuber rotting Anthracnose Brown leaf spots Cassava brown streak disease PESTS AND DISEASES PESTS 10. Fruit flies 11. Leaf cutting ants 12. Lace bugs 13. Cutworms 14. Scale insects 15. Gall midges 16. White grubs DISEASES 9. Frog skin disease 10. Witches’ broom 11. Cassava ash 12. Rust 13. Concentric ring leaf spot 14. Root smallpox disease 15. Bacterial stem gall 16. White leaf spot THRIPS Thrips (Corynothrips stenopterus) – some species damage growing points by sucking the sap of the plant resulting in loss of vigour and causes reduced yields. However it can be controlled by chemicals or insect growth regulators. CHINCH BUG Chinch Bug– this pest lives in the soil and punctures the tubers making them unmarketable. The Crotalaria plant prevents damage when planted nearby. CHINCH BUG-ROOT SMALLPOX DISEASE This problem is caused by subterranean sucking insects (Cyrtomerus bergi) which, while sucking, wounds the epidermis and cortical zone of the root with their stylet. The microorganism penetrating through these wounds cause rots. Lesions are light to dark brown. CHINCH BUG CASSAVA SHOOT FLY Cassava Shoot Fly (Siblua pendula)This pest damages the tips of the young plant causing growth to be stunted. Mature plants do not suffer as much as the younger ones. Heavy infestations occur in the beginning of the rainy season. MITES The cassava green spider mite (Mononychellus tanajoa) – this mite is green turning yellow as an adult. They occur on the lower surface of young leaves, green stems and auxillary buds and damages usually appear as yellowish “pinpricks” on the surface. Heavy attack may cause stunted and deformed leaves and complete chlorosis. Green spider mites are major pests in the dry season and severe attacks often results in 20-80% loss in tuber yield. MITES THE CASSAVA GREEN SPIDER MITE MITES The Red Spider Mite (Oligonychus gossypii) – this pest usually occur on older cassava leaves. Initial symptoms are yellowish “pinpricks” along the main vein of mature leaves which eventually turns reddish, rusty brown. Most damages occur in the beginning of the dry season. CASSAVA HORNWORM Cassava hornworm (Erinnyis ello) – high populations can defoliate large plantations in a short time. Yields are reduced and many young plants often die. HORNWORM LARVAE WHITEFLIES Whiteflies – this is the vector of the African mosaic disease. Yield losses up to 76% due to serious attack have been recorded. The whitefly sucks sap from the leaves and excrete large amounts of honeydew, which supports the growth of black sooty mould on the plant, causing premature fall of older leaves. WHITEFLIES CASSAVA MOSAIC DISEASE WHITEFLY WHITEFLIES SPIRAL WHITEFLY INFESTED CASSAVA LEAF WHITEFLIES WHITEFLIES VARIETY RESISTANT TO WHITEFLIES MEALYBUGS The cassava mealybug (Phenacoccus manihoti) – this pest is pinkish in colour and reproduces throughout the year reaching its peak during the dry season. It sucks sap from cassava shoot tips on the lower surfaces of leaves and stem during which they inject toxins into the plant causing deformation of terminal shoots which become stunted. Internodes length reduce and stems are distorted. Mealybug attack results in leaf loss and poor quality planting material. Tuber loss have been estimated up to 80%. MEALYBUG MEALYBUG The stripped mealybug (Ferrisia virgata) – this is a whitish mealybug with two longitudinal dark stripes, long glassy wax threads and two long tails. It sucks sap but unlike the cassava mealybug it does not inject any toxins into the plants therefore the attack is not as severe. The stripped mealybug occurs on the underside of the leaves near the petioles and on the stems. MEALYBUG THE STRIPPED MEALYBUG TERMITES Termites – they damage cassava stems and roots late or in the dry season. They chew and eat stem cuttings resulting in poor growth, death and rot. In older cassava plants termites chew and enter the stem which weakens them and cause breaking. GRASSHOPPERS Grasshoppers – Adults are usually dark green with yellow, black and orange markings on their bodies and the nymphs are black with yellow markings. The female lay their eggs just below the surface of the soil under the shade of the cassava tree. Eggs start to hatch at the beginning of the main dry season. They feed on cassava plants, chewing leaves and stems and may cause defoliation and debark stem. GRASSHOPPERS FRUIT FLIES The female which is yellow oviposits in the stem tissue and upon hatching the yellowish white larvae bore their way into the pith region. Heavy attacks may cause retarding of the growth and induced formation of lateral shoots. This pest does not reduce yield but rather reduce the quality of the propagation material. Yield have been reduced from 4 up to 33% when infected material were planted. FRUIT FLIES LEAF CUTTING ANTS This pest cuts semi-circular pieces of leaves and sometimes remove the buds and carry them back to their nest. Outbreaks usually occur during the first month of crop growth. Insecticides are the most effective means of control. LEAF CUTTING ANTS LACE BUGS Lace bugs attack mainly during the dry season and are severe during prolonged droughts. Adults are grey and nymphs are white both found on the underside of the leaves. Damaged leaves have small yellow spots that later turn reddish brown resembling mite damage. The rate of photosynthesis decreases and lower leaves fall. LACE BUGS LACE BUG NYMPH CUTWORMS Surface Cutworms – The black cutworm feeds on the basal part of the stem near the surface of the soil. The larvae are greasy grey to brown with faint lighter strips. Climbing Cutworms – These climb the stem feeding on the buds, foliage and stem causing the upper part of the plant to wilt and die. The larvae is dark grey to black with lateral yellow strips. CUTWORMS Subterranean cutworms – These remain in the soil feeding on the roots and underground parts of the stem. They cause damage to young plants killing up to 50%. Crickets – The black cricket or the mole cricket causes most of the damage by cutting the young shoots right after germination. CUT WORMS SCALE INSECTS This pest can cause leaves to yellow and fall out. When damage is severe, the stem is completely covered with scales causing plant growth to become stunted and stems may dessicate causing plant mortality. Heaviest outbreaks occur during the dry periods and worsen during prolonged periods of drought. Yield losses up to 20% have been recorded. SCALE INSECTS SCALE INSECTS WHITE SCALE INSECT GALL MIDGES Flies usually induce galls on cassava and are generally found in the upper surface of the leaf where they lay their eggs. The larvae causes abnormal cell growth, forming galls. The galls are yellowish green to red in colour and when they open the larvae can be seen inside. Severe outbreaks usually retard growth. GALL MIDGES WHITEGRUBS The grubs, whose adult stage is a beetle attack the cuttings and roots of cassava plants. Damages are characterized by the destructions of the bark of the cuttings and the presence of tunnels in the woody part. These cuttings may rot and die, young plants often suddenly wilt and die. WHITEGRUBS WHITE GRUB BEETLE CASSAVA BACTERIAL BLIGHT Cassava bacterial blight (Xanthomonas campestris, pv. manihotis) – heavy attack causes defoliation and stems and roots show brownish discoloration. During periods of high humidity, bacterial exudation is readily observed. This disease is spread by infected cuttings and mechanically by raindrops, use of contaminated farm tools, chewing insects and movement of man and animals through plantations. Yield loss range from 20-100%. CASSAVA BACTERIAL BLIGHT Cassava leaves showing typical Cassava plant showing shoots angular, water-soaked spots on typical of the blight stage of cassava bacterial blight the lower surface. CASSAVA BACTERIAL BLIGHT SUPERELONGATION Superelongation – there is exaggerated elongation of the stem internodes and the stem is thin, weak and longer. It may be spread by using infected cuttings. AFRICAN MOSAIC DISEASE African cassava mosaic disease (ACMD) – this disease is spread by the whitefly and infected cuttings and causes heavy yield losses of up to 90%. Symptoms occur as characteristic leaf mosaic patterns that affect discrete areas and are determined at an early stage of leaf development. Young plants have yellowish areas, frequent leaf deformation and reduced young leaf size. AFRICAN MOSAIC DISEASE BACTERIAL STEM ROT Bacterial stem rot – this is characterized by a pungent, soft stem rot and discoloration of the woody portion of the plant. Shoots of infected plants often wilt and die. Diseased cuttings used for planting do not always germinate and if they do, they are stunted with a small number of thick roots. BACTERIAL STEM ROT INTERNAL ROTTING INTERNAL ROTTING TUBER ROTTING TUBER ROT – This can be caused by pathogens that can be present in the soil or water. The predisposing factors are heavy soil, excessive flooding and poor drainage. ANTHRACNOSE Anthracnose (Colletotrichum spp. or Glomerella spp.) – this disease appears after long periods of rainfall and is characterized by the presence of leaf spots. There is partial or total necrosis of the affected tissue. The pathogen attack the green stem producing cankers and dieback. ANTHRACNOSE BROWN LEAF SPOT Brown leaf spot (Cercosoridium henningsii) – symptoms are restricted to older leaves. Brownish round spots with definite borders appear on the upper leaf surface and on the lower surface there are brownish-grey spots. Infected leaves become yellow and eventually drop. In wet areas the disease may cause yield reduction of up to 20%. BROWN LEAF SPOT BROWN STREAK DISEASE Symptoms of cassava brown streak disease on the leaves: • It appear as patches of yellow areas mixed with normal green colour commonly referred to as chlorosis. • It produces characteristic yellow or necrotic vein banding on leaves which may enlarge and join to form comparatively large yellow or necrotic patches • The infected leaves do not become distorted in shape as occurs with leaves infected by Cassava mosaic disease BROWN STREAK DISEASE BROWN STREAK DISEASE Symptoms on cassava stem – • The disease appears as dark brown “streaks” and “spots” on stems, with dead spots on leaf scars • These streaks are most prominent on upper, green portions of the stem • “Streaks” may appear as scratch-like wounds on stems • The diseased plants may show shoot tip death, which may progress into cassava stem die-back • In severe cases there may be leaf drying and shoot dieback. BROWN STREAK DISEASE BROWN STREAK DISEASE Symptoms on cassava tubers – • The disease may cause cracks and discoloration in the storage roots and often causes root constriction and malformation. • The harvested roots have corky, yellow-brown necrotic spots and root rot becomes evident. • In the roots of a susceptible variety, the disease causes a dry, hard rot which is irregular, yellow blotchy chlorosis. • Necrosis is most pronounced in the edges of the root when the root/tuber is cut across with a knife. BROWN STREAK DISEASE BROWN STREAK DISEASE FROG SKIN DISEASE This disease may cause losses from 50100%. It is characterized by the thickening of the root epidermis, thin tubers, the irregular swelling and easy breakage of the periderm and thick, cracked, wrinkled skin. This disease is transmitted by cuttings and grafting. FROG SKIN DISEASE WITCHES’ BROOM Plants affected generally produce up to 80% less than healthy plants. Symptoms include – 1. Plants that show stunting and excessive proliferation of branches, shoots have small leaves and shortened internodes without showing distortion or chlorosis. 2. Proliferation of shoots from the cuttings and are generally weak but grow without showing any visible symptoms. 3. Only a few weak and stunted shoots actually germinate from the cuttings. CASSAVA ASH This disease is common during the dry season being more prevalent on lower leaves. It is characterized by the presence of yellowish leaf spots and a white mycelium growing on the leaf’s surface. This disease is however considered to be of minor importance in yield reduction. CASSAVA ASH RUST This disease is characterized by the formation of pustules on the veins, petioles or green stems. These pustules are orange or light to dark brown depending upon the age of the pustule or the type of fungal fructification. They usually induce distortion of the affected plant. RUST CONCENTRIC-RING SPOT This disease appears during the rainy season and causes severe defoliation, dieback or total death of the plant. The spots are large and brown with no well-defined margins. CONCENTRIC-RING SPOT BACTERIAL STEMGALL The symptoms of this disease appear on the lower part of the stem and in plants older than seven months. They are characterized by galls on the stem nodes. Affected plants may be stunted and may experience dieback. WHITE LEAF SPOT Lesions are small, circular to angular, white or yellowish brown sunken from both sides, reducing the thickness of the normal leaf blade. THE MOST IMPORTANT CASSAVA PESTS AND DISEASES IN TRINIDAD AND TOBAGO 1. 2. 3. 4. 5. 6. 7. CASSAVA BACTERIAL BLIGHT. THRIPS AND MITES. SUPERELONGATION. STEM ROTS. CYDNID BUG. SHOOT FLY. TUBER ROTS. SUPERELONGATION SUPERELONGATION SUPERELONGATION SUPERELONGATION, THRIPS AND MITES THRIPS THRIPS DAMAGE THRIPS SHOOT FLY ANTHRACNOSE HORNWORM BUGS VIRUS? A NICE WAY TO END THE DAY