PPA 601 Plant Diseases and Their Management

PPA 601 Plant Diseases and Their Management eesava

WK 2 Economic Importance, Symptoms, Causal Agents, Disease Cycles and Management of Turmeric, Ginger, Taro, Cassava and Yaqona

Economic Importance

Turmeric, Ginger, Taro, Cassava & Yaqona All crops are locally grown All crops are exported to overseas markets Farmers benefit with good farm gate prices Exporters benefit with good export prices Logistic Providers benefit Consumers pay reasonable prices Exports foreign Earnings

Common Name: Tumeric Scientific Name: Curcuma longa Family: Zingerberaceae

Rhizome rot The disease is both a soil-borne and rhizomes borne and occurs with the onset of wet season (November - April). Disease symptoms: The infection starts at the collar region of the pseudostem and progresses upwards as well as downwards. The collar region of the affected pseudostem becomes water soaked and the rotting spreads to the rhizome resulting in soft rot. At a later stage root infection is also noticed. Foliar symptoms appear as light yellowing of the tips of lower leaves which gradually spreads to the leaf blades. In early stages of the disease, the middle portion of the leaves remain green while the margins become yellow. Later, the yellowing spreads to all leaves of the plant from the lower region upwards and is followed by drooping, withering and drying of pseudostems.

Survival and spread The disease is soil-borne. The fungus multiplies with build up of soil moisture with the onset of the wet weather. The fungus can survive in two ways: (a) in diseased rhizomes kept for sowing and (b) through resting structures like chlamydospores and oospores that reach the soil from infected rhizomes. Favourable conditions: Younger sprouts are the most susceptible to the pathogen. Nematode infestation aggravates rhizome rot disease. Temperature above 30 C and high soil moisture are the important predisposing factors favouring the disease. Water logging in the field due to poor drainage increases the intensity of the disease. Control: Treat the seed rhizomes with Fungicide Dithane M-45 (mancozeb 80%) for 30 minutes before sowing and also before storage. Under field conditions, drench the beds with Fungicide Dithane M-45 (mancozeb 80%).

Leaf Blotch Disease symptoms Disease symptom appears as small, oval, rectangular or irregular brown spots on either side of the leaves which soon become dirty yellow or dark brown. The leaves also turn yellow. In severe cases the plants present a scorched appearance and the rhizome yield is reduced. Spread: Soil and seed borne Survival: survive in soil on infected plant debris. Favourable conditions High soil moisture, temperature 25 O C and leaf wetness. Yield is reduced. Control: Spray Fungicide Dithane M-45 (mancozeb 80%)

Bacterial wilt Disease symptoms Rapid wilting and death of the entire plant without any yellowing or spotting of leaves are the characteristic symptom. All branches wilt at about the same time. When the stem of a wilted plant is cut across, the pith has a darkened, water-soaked appearance. Greyish slimy ooze comes out on pressing the stem. In later stages of the disease, decay of the pith may cause extensive hollowing of the stem. Favourable conditions The bacterium is especially destructive in moist soils at temperatures above 24 O C. High soil temperature and moisture are favourable for disease.

Cultural control: Soil solarisation for 60 days during summer Planting of disease-free seed rhizomes. Use crop rotation with non-host crops like rice, maize, sorghum, etc. Avoid crop rotation with tomato, potato, chillies and eggplants, as these plants are hosts for the wilt pathogen Ralstonia solanacearum. Rhizome treatment with hot water 47 O C for 3 minutes. Use bio-fumigation using cabbage and mustard plant refuses. Avoid over irrigation, and flood irrigation in disease affected areas. It is sensitive to high ph (alkaline soils).

Common Name: Ginger Scientific Name: Zingiber officinale Family: Zingerberaceae

Diseases related to Ginger Bacterial soft rot Erwinia spp. Symptoms Light yellow lower leaf tips; yellow leaves; drooping, withered leaves Cause Bacteria Comments Favors water logged fields Management Treating seed with Janola mixture prior to planting and solarizing the soil can help to reduce the incidence of the disease.

Cont d Bacterial wilt Pseudomonas solanacearum Yellowing ginger foliage caused by Bacterial wilt of ginger Symptoms Green leaves infected with the pathogen roll and curl ("green wilt"); leaves turn yellow then necrotic; plants become stunted and die; rhizomes are discoloured and water-soaked and may be rotting inside Cause Bacteria Comments Disease is spread via movement of infested soil; bacteria survive in the soil on plant debris Management Plant ginger in well draining soils where ginger had not previously been grown; plant only pathogen free seed; plant ginger on hills to aid soil drainage and promote air flow around the rhizome; rotate ginger with non-hosts of bacterial wilt.

Cont d Rhizome rots Fusarium spp. Pythium spp. Symptoms Stunted plant growth; yellow leaves and stems; brown discoloration of water conducting tissue within stem; root system rotted, mushy and turning black; rotted rhizome gives off a foul odour. Cause Fungi Comments Disease favours warm, moist soils; spread primarily through use of infected seed pieces which may not show any outward signs of disease Management Plant ginger in well-draining soils or on hills created by tilling; do not plant any seed pieces which show symptoms of disease; seed pieces can be treated with hot water (50 C - 51 C for 10 min) and appropriate fungicides prior to planting; destroy all crop debris after harvest; keep fields weed free; do not grow ginger for more than one year in same area.

Common Name: Taro Scientific Name: Colocasia esculenta Family: Araceae Subfamily: Aroideae

Taro leaf blight - Phytophthora colocasiae HOSTS: Limited, mainly aroids (Araceae), including Colocasia esculenta (taro, kalo, dasheen) and Alocasia macrorrhiza (giant taro) Symptoms and Signs Phytophthora colocasiae is primarily a foliar pathogen, but it also affects petioles and corms. The first symptoms on taro (Colocasia esculenta) are small, dark brown flecks or light brown spots on the upper leaf surface. These early spots often occur at the tips and edges of leaves where water accumulates. They enlarge rapidly, becoming circular, zonate, and purplish-brown to brown in colour.

Cont d On the lower leaf surface, spots have a water-soaked, or dry grey appearance. As spots increase in size they coalesce and quickly destroy the leaf. In dry weather, or on some resistant cultivars, the centers of lesions become papery and fall out, producing a shot-hole appearance. Dead leaves often hang on their long petioles like flags.

TLB - Pathogen Biology TLB belongs to the division Oomycota. Includes important plant pathogens, such as the downy mildews and the water moulds, Phytophthora and Pythium. Though traditionally taught along with fungi in plant pathology courses, recent studies indicate these organisms are closely related to algae and are now placed in a different kingdom.

Disease Management Cultural Practices Early disease management is aimed at reducing the inoculum level and relative humidity in the field. Taro leaf blight is an explosive disease, however, and cultural and physical control methods are usually ineffective during an epidemic. Chemical Management Protectant chemical sprays containing copper, manganese, or zinc, have been effective against taro leaf blight, but heavy rains make repeated applications necessary. Genetic Resistance Resistant cultivars offer the best long-term control of taro leaf blight. However, desirable cultural characteristics and eating qualities are often lost during breeding. Current breeding efforts therefore are focused on improving yield, suckering (desirable for vegetative propagation), time to maturity, taste, and texture.

Significance The most recent epidemic of taro leaf blight occurred in the Samoan Archipelago in 1993-1994. In 1993, the taro export market for Independent Samoa was US$3.5 million, accounting for 58% of Samoa s exports. By 1994 leaf blight had swept the islands and exported taro was valued at less than US$60,000. American Samoa, 90 km (75 miles) to the east, produced 357,000 kg (786,000 lb) of taro in 1993. This dropped to about 22,000 kg (50,000 lb) in 1994 and 5,000 kg (11,000 lb) in 1995. The severity of this epidemic was mainly due to extensive cropping with a single, susceptible cultivar throughout the archipelago

Common Name: Yaqona/ Kava Scientific Name: Piper methysticum Family: Piperaceae

Kava dieback disease Kava dieback disease is the most important cause of losses to kava production in the South Pacific. Outbreaks of the disease which have almost completely destroyed kava plantings have been observed in recent years in the main kavaproducing nations of the Fiji, Vanuatu, Tonga and Samoa. When kava plants are affected by dieback, some or all of the stems rot and die back to the stem base. The plant is either partially or completely killed. Kava dieback is thought to be caused by cucumber mosaic virus (CMV), either alone or in combination with one or more unknown agents (Davis, 1996). CMV causes important diseases of crops across the world. This virus is unusual because it has a very wide host range. It can infect over 800 different plant species. CMV is spread from plant to plant by aphids

TRANSMISSION OF THE VIRUS When aphids feed on leaves infected with CMV, they pick up the virus on their mouthparts. When these aphids move to another susceptible host plant and feed again, the aphids transmit the virus to the new plant and it multiplies and spreads within that plant. CMV is a non-persistent virus. This means that aphids pick up and transmit CMV in a few minutes and remain infectious only for several hours. The melon or cotton aphid (Aphis gossypii) is common on kava and this aphid is known to transmit CMV and kava dieback. CMV is also carried over into new plantings when stems used for planting material are cut from infected plants.

DISEASE MANAGEMENT STRATEGIES No acceptable CMV-resistant cultivars of kava have yet been found. There are no chemical sprays available which kill the virus in infected plants. Choice of planting material Disease-free planting material should be used. This will greatly reduce the number of primary infections. Stems for propagation should be selected carefully. Planting material should not be cut from plants showing CMV on the leaf or stem symptoms described above. Rouging or early harvest Infected plants should be eliminated as soon as they are detected, before the disease is allowed to spread. This is known as rouging. To rogue effectively, growers should learn to recognise early dieback symptoms and regularly inspect their crops. When rouging is timely, the root and stem base will not have rotted away and will therefore still be good for drinking.

Cont d Maintenance of traditional growing techniques The best way to grow kava is in a traditional multicrop garden in partially cleared forest. Growing kava in traditional mixed-crop gardens reduces dieback disease development. This control method takes advantage of the non-persistent nature of CMV. When aphids carrying CMV arrive in such a garden, many will feed first on the other plants and not on kava. If these plants are not hosts of CMV, the aphids lose the virus from their mouthparts and become unable to infect kava. Similarly, as aphids move through a mixed garden in which some kava dieback occurs, many which pick up CMV from infected kava will feed on a non host and lose the virus before moving to another kava plant. Suitable intercrops, which are not known to be hosts of CMV, include cassava, yams, sugarcane, ginger, paper mulberry (used for tapa/masi), vanilla, and coconut.

Common Name: Cassava Scientific Name: Manihot esculenta Family: Euphobiaceae

Cassava bacterial blight Xanthomonas axonopodis pv. manihotis Symptoms Small, angular, brown, water-soaked lesions between leaf veins on lower surfaces of leaves; leaf blades turning brown as lesion expands; lesions may have a yellow halo; lesions coalesce to form large necrotic patches; defoliation occurs with leaf petioles remaining in horizontal position as leaves drop; dieback of shoots; brown gum may be present on stems, leaves and petioles Cause Bacterium Comments Most important bacterial disease of cassava; spread by water splash and infected tools; disease more severe in wet conditions; particularly destructive in South America and Africa; most important method of spread is probably through exchange of infected plant cuttings Management Rotate cassava crop with non-host; plough crop debris into soil after harvest or remove and burn it; prune infected parts from plant; propagate cuttings only from healthy plants; intercrop cassava with corn (maize) and melon. Angular leaf spots on cassava under dry conditions; note droplets of exudate on the spots. Systemic invasion of a cassava stem leading to formation of gummy exudate and sudden wilt of the leaves (growth from a naturally infected cutting).

Transmission of Cassava Bacterial Blight - CBB The spread of the disease occurs if the bacterial causal organism is transferred from an infected plant to a new susceptible plant. The bacterium gains entry into plants through wounds or openings or scratches on leaves and stems. Contaminated Farm tools & implements - particularly cane knifes & hoes can easily be used to transmit the disease from farm to farm if tools are not cleaned after work in an infected field. Infected planting materials. The spread of the disease is largely through the use of cuttings derived from infected stems in starting new farms. Stems of CBB infected cassava plants are sources of the bacterium that causes the disease to spread.

Control of CBB Resistant Varieties In CBB endemic areas or localities with reported cases of the disease, farmers must consciously search for and plant CBB resistant varieties if economic yields of cassava are to be sustained. Rouging of Plants Farmers cultivating susceptible varieties regularly must inspect their plants and remove those showing symptoms of the disease and destroy them through burning. This practice of search and destroy must seriously follow the first few rains coming after the dry season. Early detection and destruction of plants showing symptoms of the disease can prevent or slow down the spread of the disease on a farm. Fallow If complete outbreaks on whole farms are observed, it may be necessary to harvest roots immediately and destroy stems and leaves and other plant debris through burning. Infected plant debris may also be ploughed deep into soil. In both situations the land must be allowed to fallow for few years (3 years minimum) before it is planted again to cassava. The bacterium responsible for the disease is unable to survive over a long period outside the host.

Cont d Crop Rotation In communities where land for farming is scarce, fallowing of land may not be a good disease control option. After destruction of infected plant debris as described in the preceding section, cereals, grains or legumes can be planted on the same piece of land. After three seasons the land can be returned to cassava. Quarantine Measures Restrict movement of cassava planting materials - In cases of localized outbreaks of CBB it may be necessary for a localized quarantine measure to be implemented that ensures that planting materials from CBB outbreak zones are not moved into other communities. Farmers must be educated to appreciate the importance of such a measure. Quarantine authorities must monitor movement of planting materials across borders and ensure that uncertified planting materials are quarantined to be sure of their safety. Tissue culture materials from unreliable laboratories or sources must pass quarantine regulations before they are allowed into cropping systems of countries.

Cont d Use of Healthy Planting Material Farmers must consciously harvest stems from healthy plants completely free of any of the described symptoms of the disease. It is generally wise to avoid using planting materials from any farm suspected to be under CBB attack even if symptoms of the disease have not been observed on plants.

Reference GUIDE TO IDENTIFICATION AND CONTROL OF CASSAVA DISEASES - Emmanuel Moses, John N. Asafu-Agyei, Kwame Adubofour, Augustine Adusei 2007, CSIR- Crops Research Institute, Kumasi Ghana. Kava dieback PEST ADVISORY LEAFLET NO. 25 Plant Protection Service. Secretariat of the Pacific Community. https://www.plantvillage.org H. Maraite/Universite Catholique de Louvain pictorials http://www.ctahr.hawaii.edu/oc/freepubs/pdf/c2-62.pdf http://www.kiran.nic.in/pdf/farmers_corner/pamphlets/diseases_of_ginger.pdf

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