Oryza sativa

Bacterial Leaf Blight (bacterium – Xanthomonas campestris pv oryzae: This disease was identified for the first time in Texas in 1987. The disease is very destructive in Asia, but at this time it appears that the strain of the bacterium in Texas is not very aggressive and causes negligible losses. The first symptom of the disease is a water soaked lesion on the edges of the leaf blades near the leaf tip. The lesions expand and turn yellowish and eventually grayish-white. High rainfall with strong winds are thought thought to provide conditions for the bacteria to multiply and enter the leaf through injured tissue. The bacterium has been isolated from clubhead grass (Leersia hexandra) collected form rice fields in Texas. Fall plowing or rolling of stubble to hasten decay of the rice debris should help to manage the disease by destroying the tissue in which the bacterium is maintained.

Black Sheath Rot (fungus – Gaeumannomyces graminis var. graminis): Black sheath rot or crown rot was considered a minor disease of rice in Texas for several decades, but has become increasing important with increasing intensive production systems. The fungus attacks the crown, lower leaf sheaths, and roots of the rice plant causing a dark brown to black discoloration of the leaf sheaths from the crown to considerably above the water line. As the discolored, infected sheaths decay, tiny, black, fungal reproductive structures (perithecia) form within the tissue and can be seen with a hand lens. The disease is usually observed late in the main crop season and may cause reduced tillering, poor grain fill, and lodging. Crop rotation with non-grass crops and thorough discing and maintenance of a clean fallow field from the summer prior to planting rice will help control the disease by destroying plant residue and weed host upon which the pathogen survives.

Brown Leaf Spot (fungus – Bipolaris oryzae): This disease, previously called Helminthosporium leaf spot, is common in Texas. Most conspicuous symptoms of the disease occur on leaves and glumes of maturing plants. Symptoms also appear on young seedlings and the panicle branches in older plants. Brown leaf spot is a seed-borne disease. Leaf spots may be evident shortly after seedling emergence and continue to develop until maturity. Leaf spots vary in size, are typically 1/8 inch in diameter, and are circular to oval in shape. The smaller spots are dark brown to reddish brown, and the larger spots have a darkbrown margin and reddish brown to gray centers. Damage from brown spot is particularly noticeable when the crop is produced in nutritionally deficient or otherwise unfavorable soil conditions. Significant development of brown spot is often indicative of a soil fertility problem. Brown spot may be reduced by balanced fertilization, crop rotation, and the use of high quality planting seed. Foliar fungicides are not economical for controlling brown leaf spot on most commercial long grain varieties. Rice seed with infected glumes can result in diseased seedlings. Seed treatment fungicides reduce the incidence and severity of seedling blight caused by this fungus.

Blast (fungus – Pyricularia grisea): This disease can cause serious losses to susceptible varieties during periods of blast favorable weather. Depending on the part of the plant affected, the disease is often called leaf blast, rotten neck, or panicle blast. The fungus produces spots or lesions on leaves, nodes, panicles, and collar of the flag leaves. Leaf lesions range from somewhat diamondshaped to elongated with tapered, pointed ends. The center of the spot is usually gray and the margin brown or reddish-brown. Both the shape and color of the spots may vary and resemble those of the brown leaf spot disease. Blast differs from brown leaf spot in that it causes longer lesions and develops more rapidly. The blast fungus frequently attacks the node at the base of the panicle and the branches of the panicle. If the panicle is attacked early in its development, the grain on the lower portion of the panicle may be blank giving the head a bleached whitish color, giving the term “blasted” head or rice “blast”. If the node at the base of the panicle is infected, the panicle breaks causing the “rotten neck” condition. In addition, the fungus may also attack the nodes or joints of the stem. When a node is infected, the sheath tissue rots and the part of the stem above the point of infection often is killed. In some cases, the node is weakened to the extent that the stem will break causing extensive lodging. Blast generally occurs scattered throughout a field rather than in a localized area of the field. Late planting, frequent showers, overcast skies, and warm weather favor development of blast. Spores of the fungus are produced in great abundance on blast lesions and can become airborne, disseminating the fungus a considerable distance. High nitrogen fertilization should be avoided in areas that have a history of blast. Control measures include early planting, avoiding excessive or high levels of nitrogen, proper flood management, resistant varieties, and fungicides. Varietal resistance is the most effective method of controlling rice blast. Some foliar fungicides can reduce the incidence of blast, but severe losses can occur on susceptible varieties even when fungicides are applied.

Kernel Smut (fungus – Neovossia barclayana): This disease causes losses in both yield and quality. The endosperm of the grain is attacked by the fungus causing either part or all of the starchy material to be replaced by a black mass of smut spores. The fungus does not destroy the seed embryo, and the diseased seed will germinate even if all the endosperm has been replaced. Release of the smut spores from within the kernel will cause a discoloration of hulls. Moisture causes the dark mass of spores to swell and break out of the hull. Smut is easily detected after a rain or in the early morning following a heavy dew. Kernel smut is usually most severe on late planted rice. Usually, only a small percent of the kernels are affected, but monetary losses can be significant because of penalties levied on the grain at the dryer. Milled rice has a dull or grayish appearance when smutted grains are present in the sample. High rates of nitrogen increase the incidence of smut. Varieties differ in the incidence of smut observed in commercial plantings. Less smut is detected in most commercial semi-dwarf varieties. Control measures include the use of semi-dwarf varieties in fields with a history of smut and to reduce reduce nitrogen rates and floodwater depths in fields where very susceptible varieties must be grown. The fungicide propiconazole is effective in suppressing kernel smut.

Leaf Smut (fungus – Entyloma oryzae): This is a minor fungal disease in which small slightly raised black spots develop primarily on the leaves. Raised spots or pustules break open releasing air-borne spores. Infection is often heavy enough to kill tips of leaves. Leaf smut occurs late in the growing season and causes little or no economic loss. No control measures are recommended.

Narrow Brown Leaf Spot (fungus – Cercospora janseana): The disease varies in severity from year to year and usually becomes most severe as rice approaches maturity, causing premature ripening and yield reduction. Leaf spots are long (1/10 to 1/2 inch), narrow (1/32 inch), and cinnamon-brown. Premature leaf death will occur in severe cases. Late in the growing season, the fungus often attacks the sheath of the flag leaf causing the “brown blotch” or “net blotch” phase of the disease in which a large (1 1/2 to 3 inch long) cinnamon brown lesion is formed and typically encircles the uppermost internode about an inch below the the base of the panicle. Early maturing varieties tend to escape the major impact of the disease. There are differences in suseptibility among some rice varieties, however, due to buildup of certain races of the fungus, resistance does not remain reliable. Some foliar fungicides effectively suppress this disease and may be economical if other diseases are also controlled along with the narrow brown leaf spot.

Seedling Blight and Seed Decay (fungus – Bipolaris oryzae, Pythium sp., Rhizoctonia solani, Achlya sp. and Sclerotium rolfsii): Seedling diseases cause spotty, irregular stands through seed decay, pre-emergence and post emergence disease. Seedling disease complex results from activity of various kinds of fungi, most of which grow on the kernels or hulls of seed rice or on soil organic matter. Fungi enter germinating rice seed or young seedlings and either injure or kill them. If infected seedlings emerge from the soil, they often die. Those that manage to survive are weakened and chlorotic in appearance. Damage is most severe on early seeded rice (late February and March) and deeply planted rice. Control measures include the use of high quality seed, an approved seed treatment, shallow seeding of early planted rice, and planting into warm soil.

Sheath Blight (fungus – Rhizoctonia solani): Sheath blight is perhaps the most important disease of rice in Texas. Initial symptoms usually develop as lesions on sheaths of lower leaves near the water line when plants are in the late tillering or early internode elongation stage of growth (approximately 10 – 15 days after flooding). These lesions usually develop just below the leaf collar as oval-to-elliptical, green-gray, water-soaked spots about 1/4 inch wide and 1/2 to 1 1/4 inch long. With age, the lesions expand and the center of the lesions may become bleached with an irregular tan-to-brown border. When humidity exceeds 95 percent and temperatures are in the range of 85-90 degrees F, infection spreads rapidly by means of runner hyphae to upper plant parts, including leaf blades, causing extensive, tan, irregularly shaped lesions with brown borders. Disease development progresses very rapidly in the early heading and grain filling growth stages during periods of frequent rainfall and overcast skies. Plants heavily infected at these stages produce poorly filled grain, particularly in the lower portion of the panicle. Additional losses result from increased lodging or reduced ratoon production due to infection of the culm and reduced carbohydrate reserves. As plants senesce from maturity, lesions will dry and become grayishwhite to tan with brownish borders. Sclerotia, initially white but turning dark brown at maturity, are produced superficially on or near the lesions. Sclerotia are loosely attached and easily dislodge from the plant. Sclerotia are the primary means for fungus survival between crops. They survive long periods in the soil and will float to the surface of flooded rice fields in the subsequent rice crop, infect rice plants at the waterline and continue the disease cycle. Sclerotia can survive from one to several years in the soil. They can also attack several weed hosts and cause infection. New varieties and changing cultural practices often combine many of the factors that favor disease development. In recent years, the wide acceptance of susceptible varieties, because of their high yielding potential, has contributed greatly to the rapid increase in sheath blight. In addition, the new varieties respond to heavy nitrogen applications in order to achieve their high yielding potential. Heavy applications of nitrogen predispose susceptible plants to attack by the sheath blight organism. Rotation with susceptible crops, such as soybean (see Soybean Aerial Blight) can also in crease the severity of sheath blight in succeeding rice crops. Disease incidence may be reduced by planting less susceptible varieties. None of the long grain varieties currently grown in Texas show adequate field resistance. Excessive seeding rates and high nitrogen applications should be avoided in fields with a history of the disease. Grass and weeds should be controlled. Long-term rotations may reduce the incidence of sheath blight, but soybeans, sorghum, and many weeds are susceptible to Rhizoctonia solani. Excessive seeding rates and thick plant populations favor sheath blight development. In some cases, foliar fungicides may be economical for reducing sheath blight losses.

Stem Rot (fungus – Sclerotium oryzae): Stem rot becomes most noticeable in rice fields during the latter stages of maturity. The disease occurs in circular to irregular areas in fields and causes premature death and lodging of the plants. The fungus attacks the rice plant near the water line usually during late tillering or early reproductive stages of growth. It first causes black, rectangular lesions with distinct angular borders on the leaf sheath. Later the lesions become larger, more diffuse, irregular in shape, and penetrate deep into the culm. As rice approaches maturity, injury to the stems increases and reaches its peak at harvest. Weakened stalks break during this stage and plants lodge making harvest difficult. Plants infected early yield poorly. Ratoon cropping in many areas is impractical because of the high percentage of plants killed by the disease. Diagnosis is confirmed by obtaining an infected plant, splitting the base of the stem, and observing the presence of tiny, black sclerotia in internal stem tissues. Control measures include the following: crop rotation, use of early maturing varieties, fluctuating the flood water level, avoiding excessive rates of nitrogen, and rice stubble destruction. Some fungicides help to suppress this disease but are not highly effective.

Straighthead (physiological disorder): Straighthead is a physiological disorder that causes the entire head to be blank and remain upright at maturity. Straighthead generally occurs in spots scattered throughout a field and is most easily recognized near harvest when normal plants have downturned heads (from the weight of the grain in the panicle). The disease is frequently found on sandy loam soils but seldom on clay soils. Old cotton fields with arsenic residues can have a severe incidence of straighthead. Other, as yet unknown, soil factors are also involved in causing straighthead. Often it is found in fields where excessive non-decayed vegetation has been plowed under just before planting. The disease is characterized by upright heads when the rice matures due to infertile seed. Hulls may be distorted into a crescent shape or “parrot beak”. One or both hulls may be missing. Affected plants continue to grow, are a darker green, and often produce shoots from a lower portion of the plant. First year crops of rice grown on “new ground” are more likely to be affected. Some varieties are more tolerant than others. Control measures include planting resistant varieties and draining fields with a history of the disorder just prior to internode elongation.

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