Pests and diseases (rice)

1. Bacterial Blight

Bacterial blight is caused by Xanthomonas oryzae pv. oryzae (Xoo) and affects the rice plant at the seedling stage where infected leaves turn grayish green and roll up. As the disease  progresses, leaves turn yellow to straw-colored and wilt, leading whole seedlings to dry up and die. The disease occurs in both tropical and temperate environments, particularly in irrigated and rainfed lowland areas. It is commonly observed when strong winds and continuous heavy rains occur. The disease is severe in susceptible rice varieties that are treated with high nitrogen fertilizer.

Prevention and control measures: use resistant varieties, avoid all host materials, use appropriate Nitrogen fertilizer and avoid flooding.

2.The rice blast

This disease is transmitted by a fungus called Pyricularia oryzae, which attacks the whole plant: leaves, stem, flowers and even seeds. Symptoms: They are visible especially during rainy season or in wet conditions. The disease is seen through greyish spots on stems and ears. In seedbeds, the seedlings dry out.

Period of serious outbreak of the disease: The disease is particularly severe in nursery, and at flowering stage. Chemical fertilizers with high concentration of nitrogen worsen the situation.

Prevention and control measures:

As prevention measures, plant on time and use healthy seedlings from resistant varieties; apply chemical fertilizers in moderate quantity. It is also necessary to keep the surrounding area clean, mobilize sufficient water towards the field; treat the seeds with Thiran and benomyl, (1gr of Thiran and 1gr. of benomyl per kg of seeds; soak the seeds in the solution for 24 hours before sowing in the seedbed

• Treat the disease once it is identified, using propiconazole or tebuconazole on attacked fields, and benomyl in nursery seedbeds.

Photo: Symptoms of Rice blast on leaves.

3. Rice Yellow Mottle Virus (RYMV)

Rice Yellow Mottle Virus (RYMV) is endemic and largely restricted to the African continent, where it has been found in most of the rice-growing countries. The virus has also been reported in Turkey.

Virus attacked rice plants.

Rice Yellow Mottle Virus (RYMV) is transmitted by several species of insects, cows, rats, and donkeys. Insect vectors that feed on infected wild rice usually invade and infect newly established crops or seedlings in a nursery. Rice plants, alternate hosts, ratoon or stubbles that grow between cropping seasons, allow the continuous survival of the pathogen. RYMV can also be mechanically transmitted through inter-plant movement of sap (irrigation water, guttation water, and contact between infected and healthy plant tissues and crop residues).

The virus can also been found on roots of infected plants and can infect plants through injured roots.

Symptoms:

• RYMV symptoms initially appear as yellow-green oblong to linear spots on the base of the youngest leaves. These spots later expand parallel to the leaf veins and appear as yellow or orange streaks.
• Leaves formed after infection are mottled and often twisted.
• The disease also causes brown to dark-brown discoloration and poor panicle exsertion.
• Severely infected plants are stunted, have reduced tillers, sterile spikelets, and eventually die.
• Plants infected 20 to 50 days after transplanting may show yellow stripes and spots, produce flowers and seeds, but have stunted growth.

How to manage

The use of resistant varieties appears to be the most convenient control measure against RYMV.

Other management options include:

• employ the use of large-scale synchronous planting combined with fallow period to prevent the buildup of the virus and vectors,
• plow under-infected crop residues, volunteer rice, infected ratoons, alternate hosts of the virus, and the vector immediately after harvest to reduce the primary inoculum in the field and prevent continuous survival of the virus and the vector,
• establish the crop before the increase in the vector population,
• roguing and burning infected plants, especially when infection is still low,
• regular weeding during the cropping season and even after harvest to reduce sources of primary inoculum.

4. Bacterial Leaf Streak

Symptoms begin with fine translucent streak between veins. As the disease progresses, the streaks becomes yellowish-gray, the lesion coalesce, then eventually turn to brown to grayish white causing the leaves to die.

Prevention and control measures:

• As prevention measures, plant on time and use healthy seedlings from resistant varieties; apply chemical fertilizers in moderate quantity. It is also necessary to keep the surrounding area clean, mobilize sufficient water towards the field; treat the seeds with Thiran and Benomyl, (1gr of Thiran and 1gr. of Benomyl per kg of seeds; soak the seeds in the solution for 24 hours before sowing in the seedbed.
• Treat the disease once it is identified, using Propiconazole or Tebuconazole on attacked fields, and Benomyl in nursery seedbeds.

Pests, rodents and weeds

`1. Stem borer

Larvae of the stem borer enter into the stem, feed on its heart and dig holes in the tillers. When young rice is attached by stem borers, its growth is stunting, the leaves turn yellowish and dry When rice is infected during tillering or flowering stage, grains are not well filled and become whitish. At harvesting stage, larvae start to pupate in the lower part of the stem. When the season immediately follows the harvest, stem borers increase.

Photo: Rice stem borer

Control measures:

• After harvesting, ploughing rice stubble into the soil to kill larvae and pupae and avoid leaving unwanted rations.
• Apply appropriate insecticides when deemed necessary.

2. Stalk eyed-fly

• This kind of fly with black head, called “Diopsis Thoracica’’ lays eggs which give larvae later on. These larvae attack the seedlings. This takes place mainly during dry season.

Photo: stalk-eyed fly

 

• Control measures: The field must be always clean. In addition, increase the quantity of water in the field once the first symptoms of attack by the larvae are confirmed. Finally, use specific product to fight against the fly, preferably a product containing pyrethrin.

3. Rats: rats damage rice plants in the field and can reduce the yield up to 20%.

Prevention and treatment measures:

• It is necessary to keep the surrounding area clean,
• Use rat traps and predators.

4. Birds: various birds feed in rice grains in the field and reduce the yield by 25-30%.

• Plant on time and use healthy seedlings from resistant varieties (NERICA) with sharp leaves on which birds cannot stand;
• No specific way to fight against birds other than chasing them away is used so far.

Weeds

Rice weeds which are available in Rwanda include: Echnochloa crusgali, Echnochloa Colonum, Cyperus diformis, cyperus rutondus.

Prevention and treatment measures:

• Apply chemical fertilizers in moderate quantity;
• is also necessary to keep the surrounding area clean,
• Mobilize sufficient water towards the field;
• Treat the seeds with Thiran and benomyl, (1gr of Thiran and 1gr. of benomyl per kg of seeds. Soak the seeds in the solution for 24 hours before sowing in the seedbed;
• Treat the disease once it is identified, using propiconazole or tebuconazole on attacked fields, and benomyl in nursery seedbeds.