Diseases

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Pest | Description | Crops/Host | Management | Image
Rice blast | Perfect stage: Magnaporthe grisea. Imperfect stage anamorph): Pyricularia oryzae; The fungus infects the plant by the spore germinating on the plant surface and then exerting a haustoria into the plant cells. A minimum of 8 hrs moisture is needed for infection to occur. The pathogen overwinters as spores in infected plant debris. The fungus produces new spores in the spring that re-infects rice. Spores are carried by wind and splashing rain. Movement can be over long distances. | Rice | Plant varieties resistant to blast.  Plant as early as possible within the recommended planting period. For leaf blast, re-flood if field has been drained. Maintain flood at 4 -6 inches to ensure soil is covered. Do not over fertilize with nitrogen. Apply a fungicide if necessary. | 

Sheath blight : Perfect stage:  Thanatephorus cucumeris (A.B. Frank) Donk. Imperfect stage (anamorph): Rhizoctonia solani Kuhn | R. solani can infect rice from the seedling stage to harvest. The pathogen overwinters as small seed like structures called sclerotia or as mycelium in infected plant debris. This material floats in the first flood and comes in contact with the rice plants. It tends to accumulates toward one side of the field due to wind. | Rice | Plant varieties less susceptible to sheath blight. Plant as early as possible within the recommended planting period. Avoid late planting. Avoid excessive plant stands. Do not over fertilize with nitrogen.  Apply a fungicide if necessary. | 

Seedling disease: Caused by: Cochliobolus miyabeanus (Ito & Kuribayashi) Drechs. Ex Dastur, Curvularia spp., Fusarium spp., Rhizoctonia solani Kuhn, Sclerotium rolfsii Sacc. (teleomorph: Athelia rolfsii(Curzi) Tu & Kimbrough), and other pathogenic fungi | Whitish outgrowths of fungal mycelium emerge from cracks in the seed glumes or from the collar of the infected seedling’s plumule. After a few days, the fungal mycelium resembles a halo that radiates from the infection point on the seed or seedling. If seedlings produce primary leaves and roots before infection occurs, they usually survive but are typically stunted. Leaves and sheaths become yellow or chlorotic, and further development is retarded. | Rice | Seed treatments with Mancozeb or Ridomil increase stands by 20-30%. The practice of pin-point flooding helps reduce water-mold damage. Seeding should not begin until the mean daily temperature reaches 65 degrees F. |

Brown spot disease: Causative Agent: Cochliobolus miyabeanus Synonyms Ophiobolus sativus, Helminthosporium oryza C. miyabeanus is an ascomycete, a member of the sac fungi. | It is a member of a group of pathogens that attack members of the grass family, including important cereal species such as wheat, maize, barley, oats and rye. The disease is first seen as brownish spots on the leaves and glumes of the plant. The spots enlarge and become grey at the center and brown at the edge. The affected tissues take on a velvety feel as the fungus begins to develop aerial structures that produce the spores by which it spreads. The spores are carried in the rice seed and when it germinates, seedlings are weakened and crop yield is reduced. | Rice | Plants growing in good nutritional conditions are generally resistant to the disease. Rice plant becomes susceptible when silicon is deficient. Preventive treatment of fields with calcium silicate would be useful if a threat was present. |

HLB : Citrus huanglongbing (HLB) Disease known by Chinese name (formerly known as Citrus Greening Disease (CGD)) is one of the most devastating disease of Citrus. The HLB disease is caused by a phloem-restricted bacterium, Liberibacter spp. In Asia, so far the disease is known to be only caused by Liberibacter asiaticus that is vectored by a psyllid, Diaphorina citri. The species that causes HLB disease in Africa is Liberibacter africanus vectored by another psyllid, Trioza erytreae. The Asian liberibacter and D. citri are both heat tolerant, and occur even at low altitudes. The African liberibacter as well as T. erytreae are heat sensitive, and occur only in relatively cool areas, such as those above altitudes of 1000m or so. | HLB-affected trees often show only one branch with symptomatic, yellow and mottled leaves.  Such a yellow branch shows up conspicuously within the green canopy of the tree. Fruits on HLB infected trees show color inversion with the presence of brownish, aborted seeds in the fruit. Normal fruits break color and turn orange first at the stylar end, the peduncle end being still green. HLB affected fruits become orange first at the peduncle end, the stylar end being still green. HLB-affected mandarins are sometimes lopsided and usually taste bitter compared to normal fruit. | Citrus | Management activities outlined below must be carried out to manage HLB. HLB Surveillance: It is vital that the Extension staff routinely monitor for HLB disease and its vector.  Orchards should be surveyed to identify trees that are infected with HLB pathogen by observing for signs and symptoms of the HLB disease. The part of trees that show symptom must be removed and destroyed through burning. Severely declining trees that are uneconomic must be uprooted and burned as well. Vector Control: Vector monitoring and its control should be one strong component of the whole HLB management plan. Hence routine monitoring of the vector must be carried out to plan and carry out timely control measures as recommended. Replacement of uprooted trees: For replacing uprooted trees, healthy disease free seedling must be used while still adopting vector control and removing affected parts of a trees (or severely affected trees). The young seedlings that are replanted should be monitored for symptom development. If disease symptom is observed, then it is best that the seedling be removed and burned. Orchard Sanitation: Periwinkle is a preferred host for the HLB pathogen. However, it requires transmission through dodder plants. For the Psyllid vector the preferred host for development and multiplication is Murrya paniculata. Therefore, these two hosts of the vector and pathogen and the dodder plant must be eradicated from orchards and in the vicinity. |

Powdery mildew At least two species of PMs occur on citrus: tingitaninum possibly rare citri/Erysiphe euonymi-japonici probably common throughout the world. Possibly both species occur on the subcontinent. citri/Erysiphe euonymi-japonici occurs on many plant species including species of Citrus and Citrus relatives. The species can be distinguished based on the morphology of conidia and appressoria. | Overwinters as mycelium in buds. Shoots are most susceptible to infection. Germ tubes place haustoria in  epidermal cells – do not penetrate via stomata. Spores germinate within 7 h. Germination stimulated by light. Spores do not germinate in water. High RH favours germination. Optimum temperature for germination ~20ºC. | Citrus | Spray plants when new flush is developing: Treatments: 1. Water. 2. Carbendazim at manufacturer’s rate 3 sprays at 2 week intervals. 3. Tridemorph at manufacturer’s rate 3 sprays at 2 week intervals. 4. Wettable sulphur at 0.15% for the first application and then 2 sprays at 2 week intervals until flush is mature. 5. Prune infected shoots and burn them. |  

Scab: Scab caused by fungus Elsinoe fawcetti | Scab widely distributed occurring where rainfall conditions are conducive to infection: Shoot infection cause stunting of seedlings in nurseries. Scab affects production of fruit for the fresh market. Infection of young fruit promotes formation of hyperplasic tissue; When first formed, scab spot range from pink to light brown. Later they become corky and turn yellowish or grayish brown or even black. Scab fungi over-winter on the tree canopy. Scab fungi requires a minimum of 2.5 hrs. of continuous wetting for the germination of conidia and infection. Temperature does not have a major impact on disease severity. | Citrus | Several fungicide spray treatments needed to control scab on foliage in nurseries.  In orchards, spray treatments to prevent fruit infection; Earlier treatment during spring flush to prevent buildup of inoculum and reduce subsequent disease pressure on fruit. |

Loranthus  belongs to a family of mistletoes parasitic on trees and shrubs | Birds spread the parasite; In grossly neglected orchards mandarin tees can die of infestations by Loranthus. Species attacking mandarin trees is Scurrula parasitic L.; Scurrula parasitica also attack conifers like Chir Pine and other deciduous trees, including Apple.| Citrus | Pruning parasite from infested branches. All wood that is touched by the parasite should be pruned out. Glyphosate show some effect, particularly on juvenile mistletoes, when injected at rates of 10 ml per tree. | 

Phytophthora Rot: caused by fungi Phytophthora citrophthora and P.  parasitica | Phytophthora foot rot and gummosis: most serious soil borne diseases.  P. citrophthora: frequently attack aerial part; Infection usually occurs through zoospores; Zoospores enter plant through wounds or natural openings; Phytophthora spp. can survive unfavourable conditions in the soil or in decayed organic matters. | Citrus | Usually the disease is confined to the feeder roots and is not very noticeable. Infected trees have fewer and smaller leaves. Leaves: Chlorosis starts first in the midrib and then on the lamina. Leaves then fall off and twigs and branches begin to die back. |

Maize leaf Blights | Northern leaf blight caused by Helminthosporium turcicum is also known as Turcicum leaf blight and this disease is known to have caused the American famine of 1970. Southern leaf blight also known as Maydis leaf blight is caused by Bipolaris maydis, syn. Helminthosporium maydis. These diseases are known to cause economic losses of the crop. Northern leaf blight is prevalent under moderate and high humidity conditions. Southern corn blight occurs in hot, humid areas. However, both species can be commonly found on the same plant also. | Maize | Early detection of the diseases is important to prevent crop major crop losses. Regular monitoring will help detect diseases on time: monitor filed at least every four week from whorl through dent stage. Frequent monitor (once in 2 weeks) may be necessary during prolonged wet and warm weather condition. Monitoring methodology: 1. During each monitoring, randomly select observation sited depending on the size of the filed. g., at least two sites for one acre sized filed. Do not select form border. 2. Within the observation site, observed 3-4 rows of both 20ft in length or 20 plant per site. 3. If the diseased lesion are observed on the lower leaves especially when plants are flowering stage, monitoring must done at least every week. Fungicide spray must initiated immediately if the lesion are observed to be spreading towards the ear leaf. 4. Our famers grow susceptible varieties every year under minimum till condition, monitoring is the key in managing diseases on time. Cultural control: 1. Infected maize stubbles are source of inoculum for next growing season. Therefore stubbles should be plough in right after harvest for decomposition. 2. Avoid the planting in same area if possible. For GLS the fungus is not known to survive more than two years. 3. Use tolerant varieties like yangtsepa and Khangma ashom 1 & 2 | 

Phytophthora blight (LB) | Occurs in the potato as well as tomato. The fungus can survive in infested tuber (potato) and tomato seeds and in the soil. Symptoms: Grayish dark spots appear on leaves, stem, and branches and in tomato even on both green and ripe fruits.  Undersides of infected leaves and other infected parts develop a white mold during moist conditions. Infected tubers show irregular, small to large, slightly depressed areas of brown to purplish skin. Boundary between diseased and healthy tissue is not distinguished. | Potato | Early potato plantation usually escapes disease. Spray Mancozeb when LB is first detected and repeat two-three times depending on weather conditions. Cut the infected haulms to avoid seed infection (infected tubers are the main source of infection in potato). Sort diseased tubers before storage/Avoid taking seeds from infected plants. Collect and bury (deep) infected tubers and vines. Remove volunteer plants immediately after emergence. |

Stripe Rusts of Wheat and Barley. Pathogen: Puccinia striiformis | Different races (strains) of the stripe rust pathogen affect wheat and barley. The stripe rust fungus has been responsible for some of the most devastating epidemics on wheat. The fungus grows only on living host plants and survives between seasons on volunteer wheat, barley plants, and some wild grasses. Rust spores are spread by wind to initiate infections. Disease development is most rapid at temperatures of 10° to 16°C with intermittent rain and dew, although disease can continue to develop at higher temperatures and drier conditions; secondary cycles occur at 7- to 10-day intervals. Races infecting barley can survive in warmer, drier climates. Infections increase water loss and decrease the amount of photosynthate available for grain filling, resulting in reductions in the number and weight of kernels. | Wheat and Barley | Control is achieved through the use of resistant cultivars. A statewide monitoring program exists for early detection of susceptible genotypes and identification of new resistant genotypes. Chemical Control: In the event that new races of the fungus render current sources of resistance obsolete, foliar fungicides can be applied to control disease outbreaks. Application timing will depend on when initial infections occur; the objective is to protect the flag leaf from infection and to protect the plant during the grain-fill period. |                                                                    

Phytophthora Blight :  Phytophthora capsici | Phytophthora capsici infects roots, crowns, stems, leaves, and fruit, causing seedling damping-off, stem lesion, stem blight, leaf spot, and fruit rot. The first symptom on pepper in the field is commonly crown rot. A lesion girdling the base of the stem causes rapid collapse and death of the plant. This phase of the disease usually occurs in low-lying areas of the field. Following rainstorms, black, girdling lesions form on the stem and in the exiles of chili branches, resulting in wilting of leaves and branches. The affected plants gradually die. It is not uncommon for an entire field to exhibit plant blight. When leaves are infected, half-moon shaped, tan lesions form at the margin. Infected fruit develop dark, water-soaked lesions, which are commonly covered with white mold. The water-soaked lesion may not be immediately obvious; however, the soft, infected tissues may slip off, or can be easily punched when touched during picking. Affected fruit wither, but remain attached to the plant. | Potato | No single strategy should be used to control Phytophthora blight. A combination of methods is needed to effectively control this disease. Strategies recommended for management of Phytophthora blight include preventing the pathogen from being moved to the field, reduction of soil moisture, reduction of Phytophthora spores in the soil, utilization of resistant varieties, and applying fungicides. Select fields with no history of Phytophthora blight, if possible.  Select well-drained fields. Do not plant the crop in the low areas or the areas which do not drain well. Clean farm equipment of soil between fields. Plant on raised beds (a minimum of 9 inches high. Do not irrigate from a pond that contains water drained from an infested field.  Scout the field for the Phytophthora symptoms, especially after major rainfall, and particularly in low areas. As Phytophthora symptoms become obvious, remove infected plants to reduce the amount of spores produced.  Do not save seed from a field where Phytophthora blight occurred. Fungicides may be used to reduce Phytophthora infection. Chemical control strategies should be integrated with cultural practices to manage Phytophthora blight. Chemical control measures commonly involve soil drenches early in the season, and foliar applications during the season. |  [/table]