Padi Pathogens Malaysia
Padi Pathogens in Malaysia.
The major pathogens to be tested with our products are (a) Brown Spot; (b) Leaf Blast; (c) Leaf Blight; and (d) Sheath Brown Rot.
> BROWN SPOT.
Brown Spot has been historically largely ignored as one of the most common and most damaging rice diseases.
What it does - Brown spot is a fungal disease that infects the coleoptile, leaves, leaf sheath, panicle branches, glumes and spikelets. Its most observable damage are the numerous big spots on the leaves which can kill the whole leaf. When infection occurs in the seed, unfilled grains or spotted or discolored seeds are formed.
Why and where it occurs - The disease trives in high relative humidity (86 - 100%) and temperatures between 16°C and 36°C. It is common in un-flooded and nutrient-deficient soil, or in soils that accumulate toxic substances. For infection to occur, the leaves must be wet for 8 - 24 hours. The fungus can survive in the seed for more than 4 years. It can spread from plant to plant through air. Major sources of brown spot in the field includes : (a) infected seed, which leads to infected seedlings; (b) volunteer rice; (c) infected rice debris and (d) weeds. Brown spot can occur at all crop stages, but infection is most critical during maximum tillering up to the ripening stages of the crop.
How to identify - Check for lesions. Infected seedlings have small, circular, yellow brown or brown lesions that may girdle the coleoptile and distort primary and secondary leaves. Starting at tillering stage, lesions can be observed on the leaves. They are initially small, circular, and dark brown to purple-brown. Fully developed lesions are circular to oval with a light brown to gray center, surrounded by a reddish brown margin caused by the toxin produced by the fungi. On susceptible varieties, lesions are 5 - 14 mm long which can cause leaves to wilt. On resistant varieties, the lesions are brown and pinhead-sized.
Lesions on leaf sheaths are similar to those on the leaves. Infected glumes and panicle branches have dark brown to black oval spots or discoloration on the the entire surface. Spikelets can also be infected. Infection of florets leads to incomplete or disrupted grain filling and a reduction in grain quality. The disease-causing fungi can also penetrate grains, causing “pecky rice”, a term used to describe spotting and discoloration of grains. Spots and lesions on leaves. In certain rice varieties, brown spot lesions can be mistaken for blast lesions. To confirm, check if spots are circular, brownish and have a gray center surrounded by a reddish margin.
Why is it important - Brown spot causes both quantity and quality losses. On average, the disease causes 5% yield loss across all lowland rice production in South and Southeast Asia. Severely infected field can have as high as 45% yield loss. Heavily infected seeds cause seedling blight and lead to 10 - 58% seedling mortality. It also affects the quality and the number of grains per panicle, and reduces the kernel weight. Brown spot was considered to be the major factor contributing to the Great Bengal Famine in 1943.
How to manage - Improving soil fertility is the first step in managing brown spot. To do this : (a) monitor soil nutrients regularly; (b) apply required fertilizers and (c) for soils that are low in silicon, apply calcium silicate slag before planting. Fertilizers, however, can be costly and may take many cropping seasons before becoming effective. More economical management options include : (a) Use resistant varieties; (b) Contact your local agriculture office for up-to-date lists of varieties available; (c) Use fungicides (e.g., iprodione, propiconazole, azoxystrobin, trifloxystrobin and carbendazim) as seed treatments and (d) Treat seeds with hot water (53 − 54°C) for 10 − 12 minutes before planting, to control primary infection at the seedling stage. To increase effectiveness of treatment, pre-soak seeds in cold water for 8 hours.
> LEAF BLAST.
What it does - Blast is caused by the fungus Magnaporthe oryzae. It can affect all above ground parts of a rice plant : leaf, collar, node, neck, parts of panicle and sometimes leaf sheath.
Why and where it occurs - Blast can occur wherever blast spores are present. It occurs in areas with low soil moisture, frequent and prolonged periods of rain shower and cool temperature in the daytime. In upland rice, large day-night temperature differences that cause dew formation on leaves and overall cooler temperatures favor the development of the disease. Rice can have blast in all growth stages. However, leaf blast incidence tends to lessen as plants mature and develop adult plant resistance to the disease.
How to identify - Check the leaf and collar for lesions. Initial symptoms appear as white to gray-green lesions or spots, with dark green borders. Older lesions on the leaves are elliptical or spindle-shaped and whitish to gray centers with red to brownish or necrotic border. Some resemble diamond shape, wide in the center and pointed toward either end. Lesions can enlarge and coalesce, growing together, to kill the entire leaves.Blast lesions can commonly be confused with Brown Spot lesions. Leaf blast lesions are usually elongated and pointed at each end, while brown spot lesions tend to be more round, brown in color and have a yellow halo surrounding the lesion.
Why is it important - Rice blast is one of the most destructive diseases of rice. A leaf blast infection can kill seedlings or plants up to the tillering stage. At later growth stages, a severe leaf blast infection reduces leaf area for grain fill, reducing grain yield. Leaf blast can kill rice plants at seedling stage and cause yield losses in cases of severe infection.Collar blast Old lesions Spindle lesions Diamond lesions
How to manage - The primary control option for blast is to plant resistant varieties. Other crop management measures can also be done, such as : (a) Adjust planting time. Sow seeds early, when possible, after the onset of the rainy season; (b) Split nitrogen fertilizer application in two or more treatments. Excessive use of fertilizer can increase blast intensity and (c) Flood the field as often as possible. Silicon fertilizers (e.g., calcium silicate) can be applied to soils that are silicon deficient to reduce blast. However, because of its high cost, silicon should be applied efficiently. Cheap sources of silicon, such as straws of rice genotypes with high silicon content, can be an alternative. Care should be taken to ensure that the straw is free from blast as the fungus can survive on rice straw and the use of infected straw as a silicon source can spread the disease further. Systemic fungicides like triazoles and strobilurins can be used judiciously for control to control blast. A fungicide application at heading can be effective in controlling the disease.
> LEAF BLIGHT.
What it does - Bacterial blight is caused by Xanthomonas oryzae. It causes wilting of seedlings and yellowing and drying of leaves.
Why and where it occurs - The disease is most likely to develop in areas that have weeds and stubbles of infected plants. It can occur in both tropical and temperate environments, particularly in irrigated and rain-fed lowland areas. In general, the disease favors temperatures at 25 - 34°C, with relative humidity above 70%. It is commonly observed when strong winds and continuous heavy rains occur, allowing the disease-causing bacteria to easily spread through ooze droplets on lesions of infected plants. Bacterial blight can be severe in susceptible rice varieties under high nitrogen fertilization.
How to identify - Check for wilting and yellowing of leaves, or wilting of seedlings (also called kresek). On seedlings, infected leaves turn grayish green and roll up. As the disease progresses, the leaves turn yellow to straw-colored and wilt, leading whole seedlings to dry up and die. Kresek on seedlings may sometimes be confused with early rice stem borer damage. To distinguish kresek symptoms from stem borer damage, squeeze the lower end of infected seedlings between the fingers. Kresek symptoms should show yellowish bacterial ooze coming out of the cut ends. Unlike plants infested with stem borer, rice plants with kresek are not easily pulled out from soil. Lesions caused by bacterial blight. Check for lesions. On older plants, lesions usually develop as water-soaked to yellow-orange stripes on leaf blades or leaf tips or on mechanically injured parts of leaves. Lesions have a wavy margin and progress toward the leaf base. On young lesions, bacterial ooze resembling a milky dew drop can be observed early in the morning. The bacterial ooze later on dries up and becomes small yellowish beads underneath the leaf. Bacterial ooze Dried up bacterial ooze. Old lesions turn yellow to grayish white with black dots due to the growth of various saprophytic fungi. On severely infected leaves, lesions may extend to the leaf sheath. To quickly diagnose bacterial blight on leaf : (a) cut a young lesion across and place in a transparent glass container with clear water and (b) after a few minutes, hold the container against light and observe for thick or turbid liquid coming from the cut end of the leaf.
Why is it important - Bacterial blight is one of the most serious diseases of rice. The earlier the disease occurs, the higher the yield loss. Yield loss due to bacterial blight can be as much as 70% when susceptible varieties are grown, in environments favorable to the disease. When plants are infected at booting stage, bacterial blight does not affect yield but results in poor quality grains and a high proportion of broken kernels.
How to manage - Planting resistant varieties has been proven to be the most efficient, most reliable, and cheapest way to control bacterial blight. Other disease control options include : (a) Use balanced amounts of plant nutrients, especially nitrogen; (b) Ensure good drainage of fields (in conventionally flooded crops) and nurseries; (c) Keep fields clean. Remove weed hosts and plow under rice stubble, straw, rice ratoons and volunteer seedlings, which can serve as hosts of bacteria and (d) Allow fallow fields to dry to suppress disease agents in the soil and plant residues.
> SHEATH BROWN ROT.
What it does - Sheath brown rot is caused by Pseudomonas fuscovaginae. It causes rotting in sheaths and grains of seedlings and mature plants.
Why and where it occurs - Sheath brown rot can occur in areas of high altitude (1,200 - 1,700 m above sea level), low temperature (20 - 22°C), and high humidity, in both temperate and tropical conditions. The disease is known to particularly occur in Asia. The disease is seed borne and its causal bacteria, P. fuscovaginae, survive in the field as epiphyte to growing rice plants or weeds.
How to identify - Check for discoloration on the leaf sheath. Symptoms typically occur on the flag leaf sheath at booting stage and on the panicle. At seedling stage, a systemic discoloration occurs, which may spread to the midrib or veins of the leaves. Infected seedlings initially show yellow to brown discoloration on the lower leaf sheath. When plants mature, oblong to irregular dark green, water soaked lesions occur, which later turns gray-brown or brown, and may be surrounded by an effuse dark brown margin. The sheath may also exhibit general water-soaking and necrosis without definable margins. With severe infections, the entire leaf sheath may become necrotic and dry out, and the panicle withers. Glumes emerging from infected plants exhibit water-soaked lesions that turn light brown. Grains of infected panicles are discolored, deformed, or empty.
Why is it important - Although not common, yield losses as high as 72.2% due to bacterial sheath brown rot were reported in Indonesia. Severe losses were also reported in rice production areas in Central Africa and Madagascar. Under very severe infection, total yield loss (almost 100%) was observed in Madagascar.
How to manage - Since the disease is seed borne, preventive measures are very important : (a) Clean the field immediately after harvest, and off-season cultivation of a crop. Make sure to remove plant litter and re-growths; (b) Adjust sowing time to avoid low temperatures; (c) Use seedlings that are 20−30 days old rather than older seedlings; (d) Use clean seeds and (e) Treat seeds with hot water at 65°C.