RICE DISEASES AND THEIR MANAGEMENT/ Mary Goretti
The major rice diseases in Kenya is Rice Blast and other important diseases: Bacteria leaf Blight , Brown spot, Alternaria leaf spot , Stem Rot and Yellow mottle virus and some of the symptoms appear below
1. Rice blast
Distribution: all rice growing ecologies in Kenya.
Lesions appear on leaves; the tips of lesion are typically spindle-shaped wide in the centre and pointed at the ends. Large lesions usually develop a grey centres bordred by brown to dark red margins.
Use of resistant varieties Use clean seed for planting
Use recommended spacings (20 x 20cm) at planting to enhance aeration and light penetration
Use recommended dozes of nitrogen for the respective area.
Maintain clean fields as sone weeds (wild rice, eleusine spp and other grases) are alternate hosts which can be sources of infection.
Do not work in the field when it is too wet
Burn or bury infected crop debris.
Fungicide application 2 vegetative stage (35DAT) and Booting Stage (70DAT)
- Carbendazim (Rodazim, Goldazim , Bavistin),
- Probenazole, Tricyclazone, Benomyl, Topsin,
- Quadris 2.08 SC (azoxystrobin
2. Bacterial leaf spot
Yellow to white water soaked stripes at the margins of infected leaves.
Later the stripes enlarge and coalesce to give the characteristic yellowish lesions mainly in the upper part of the affected leaves. At times the entire leaf may turn whitish to greyish and die. In severe cases the whole field seems to have been burnt
The disease starts from infected seeds. It is spread between plants and fields by wind, rain, irrigation, water floods.
Use resistant varieties
Observe field sanitation - removal of weeds which can be carriers, and previously infected crop residues.
3. Brown leaf spot
Disease start as small, circular, dark brown or purplish dots onleaves. Fully developed lesions are brown, with grey or whitish centres. Most spots have a light yellow halo around.
- Source of disease in the field is infected seeds. Secondary infection occur through airborne spores.
- Planting clean seeds or seed treatment with fungicidal
- chemicals (Dithane M45 and ceresan; Benlate T; 50% copper
- oxychloride etc) can give complete control.
- Brown spot is favoured under poor soil condition.
Proper application of fertilizers, good water management and soil amendments can reduce disease severity.
Resistant varieties are available and can be used
4. Rice yellow mottle virus
Leaves turn yellow, with alternate yellow and green stripes that give its typical mottled appearence to the plant. Other symptoms are stunting, reduced tillering, leaf mottle with yellow stripes, incomplete anicle insertion, badly formed panicle and spikelet sterility.
Use of resistant varieties such as Resistance varieties- Basmati 370 and 217 , ITA 310
Destroy wild rice and grass weeds which are alternate horsts.
Avoid continous rice cropping – Preferably plant other crops during off season to avoid building up of vectors.
5. Sheath rot
Sheath rot symptom
Infection occurs on the uppermost leaf sheath at late booting stage. Symptoms start as irregular spots with gray centres and brown margins or gray brown throughout.
Severe infection cause panicles to be only particially inserted, rotted and produce poorly filled grains. Powdery fungal growth is seen inside the leaf sheath.
INTEGRATED MANAGEMENT OF RICE DISEASES
Integrated rice disease management (IDM) is the combination of different methods to control diseases in a cost effective way, based on sound environmental management. None of the methods can by itself ensure efficient and sustainable protection.
The methods often combine varietal resistance, crop management techniques and modest use of chemicals.
Integrated disease management strategies
Appropriate sowing or transplanting date to avoid flowering Coinciding with high atmospheric humidity which is favourable to blast infestation.
Syncronized sowing and transplanting to avoid build up of insect pests that can transmit diseases such as RYMV.
Using heathy or treated seed – fungal and bacterial diseases can be transmitted via seed.
Destruction of weeds and crop residues that can act as reservoirs for pathogens and insect vectors.
- Balanced and reasonable fertilizer use: High dozes of nitrogen can increase blast infestation, while high dozes of potassium generally limit infestation.
- Destruction of rice stubbles and vector host plants to avoid pathogen build up and to interrupt the life cycles of insect vectors.
- Cleaning of canals and borders of plots that can be reservoirs of RYMV, alternative hosts and insect vectors.