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Scab

Scab is a disease of potato tubers that results in lowered tuber quality due to scab-like surface lesions. There are no above-ground symptoms. Two forms of scab occur. Common scab occurs in all production areas and is most severe in soils with a pH above 5.5. Another less common form, called acid scab, is important in acidic soils (below pH 5.5).

Symptoms

Scab symptoms are quite variable. Usually, roughly circular, raised, tan to brown, corky lesions of varying size develop randomly across tuber surfaces. Sometimes scab develops as a rather superficial layer of corky tissues covering large areas of the tuber surface. This is called russet scab. Pitted scab can also occur where lesions develop up to 1/2 inch deep. These deep lesions are dark brown to black, and the tissues underneath are often straw-colored and somewhat translucent. More than one of these lesion types may be present on a single tuber. Although scab symptoms are usually noticed late in the growing season or at harvest, tubers are susceptible to infection as soon as they are formed. Small brown, water-soaked, circular lesions are visible on tubers within a few weeks after infection. Mature tubers with a well-developed skin are no longer susceptible, but existing lesions will continue to expand as tubers enlarge. Thus disease severity increases throughout the growing season. Scab is most severe when tubers develop under warm, dry soil conditions. Coarse-textured soils that dry out quickly are therefore more conducive to scab than are fine-textured soils.

A few other conditions can be confused with scab. White, enlarged lenticles, which frequently occur on potato tubers harvested from wet soil, can be mistaken for scab. Usually this condition will disappear when tubers are dried. Patchy russeting, checking, or cracking of tuber surfaces caused by the fungus Rhizoctonia also may be confused with russet scab. A totally different but uncommon disease called powdery scab, caused by the fungus Spongospora subterranea, causes very similar scab-like symptoms. Laboratory examination may be necessary to identify these diseases.

Causal Organisms

Scab is caused by a group of filamentous bacteria called actinomycetes that occur commonly in soil. In soils with a pH above 5.5, Streptomyces scabies is usually responsible for common scab, and is capable of causing all the types of scab lesions described above. It is commonly introduced into fields on seed potatoes, and will survive indefinitely on decaying plant debris once the soil is contaminated. Because the organism can survive passage through the digestive tract of animals and be distributed.

Head blight or scab is an important disease of wheat, oats, barley and spelt in years when warm, wet weather persists during the heading and blossoming period of these cereal crops. Scab is not only a disease of small grains, but of many grasses including wheat, foxtail, quackgrass, crabgrass and bluegrass.

Scab is only one of several diseases of small grains caused by species of Fusarium. Seedling blight, due to planting scabby kernels, is an important disease effectively controlled by seed treatment fungicides. The Fusarium fungi also cause root rot and crown rot diseases in small grains. Corn is an important host for these fungi in the Corn Belt. On corn these fungi cause ear, stalk and root rot diseases.

The severity of scab infection varies greatly from year to year. Severe infection occurs during the flowering stage and shortly afterward when wet weather prevails. Two to three days of light rain during this period will initiate epidemics. If the weather is dry during this critical period, the grain crop will be essentially scab-free.

In Ohio, during years of favorable weather, the incidence of infected heads has been as high as 100 percent in some fields. In these cases over 50 percent of the spikelets have been destroyed. Other affects of scab include floret sterility, poor test weights due to shriveled grain and yield loss. In general, oats are less susceptible to scab than either wheat or barley.

Figure 1. Wheat head showing bleached out florets affected by scab. Note salmon pink spore mass on the edge of affected glumes.

Wheat head showing bleached out florets affected by scab

Scab is important, not only because it reduces yield, but it reduces the quality and feeding value of the grain. The fungi causing scab may produce chemicals in the infected grain which are toxic to livestock and humans. Vomitoxin, or deoxynivalenol, contamination has been a problem in wheat in Ohio during years with scab epidemics.

Symptoms

The earliest and most conspicuous symptom of scab occurs soon after flowering. Diseased spikelets turn light-straw colored and have a bleached appearance due to premature death of tissues. Healthy spikelets on the same head retain their normal green color. One or more spikelets may be infected, or the entire head may be diseased. When the fungus infects the stem immediately below the head the entire head may die. Infected spikelets of oats are ash-grey and those of barley are light brown.

Several days after infection masses of pink to salmon-colored spores and mycelium may form on the margin of the glumes of individual spikelets, especially near the base of the kernel. The pink spore masses are easiest to see early in the morning before the dew dries. Infected kernels are generally shrunken, wrinkled, and light in weight, with a rough, scabby appearance. These kernels range in color from light-brown to pink to grayish white. The extent of shriveling and discoloration of the kernels depends on the time of infection and the weather conditions following infection.

If the fungus invades and kills the rachis or main axis of the spike, the spikelets above that point die. The result is no grain at all or small, shriveled kernels that are lost during the threshing process. Heads with diseased spikelets may become speckled with dark purplish-black fruiting bodies (perithecia) of the fungus if the weather remains cool and moist until harvest. These perithecia are a sign of the sexual stage, the Gibberella stage of the fungus.

Disease Cycle

The fungi causing scab are all members of the genus Fusarium, the asexual spore forming stage. The principal pathogen is Fusarium graminearum (the sexual stage is Gibberella zeae), but F. avenaceum (G. avenaceum) and F. culmorum (no known sexual state) have also been reported.

The scab fungi overwinter and survive between crops in infected grain and grass stubble, chaff, and cornstalk residue left on the soil surface. They survive as asexual spores (conidia), mycelium, and perithecia within which are borne the sexual spores (ascospores). The fungi are also common contaminants on seed. These fungi continue to grow and produce spores from harvest until the residues decompose in the soil.

The conidia are produced profusely during warm, moist weather on corn and small grain residues. Ascospores produced within perithecia are discharged into the air as the fruiting bodies wet and dry during fluctuations in moisture levels. Air currents carry these spores to the flowering spikelets. Ascospores and conidia have been collected in the air above wheat fields, and both are capable of causing infections. The spores germinate in free water on the surface of the spikelet and invade the flower. Infections are most serious when the anthers are exposed during flowering. Symptoms develop in as little as three days after infection when temperatures range from 77 to 85 degrees F (25-30 degrees C) and humidity is high. Within 7 to 10 days after infection, salmon-pink masses of conidia form at the base of the diseased spikelets. These conidia can be blown by the wind to the heads of other cereal or grass plants producing new secondary infections. This process continues to be repeated as long as the spikelets are susceptible and moist weather prevails. Secondary infections may occur from long distance spread of air borne conidia. Ascospores are usually produced on the heads too late in the season to function as secondary inoculum. However, ascospores may persist in crop residues and contaminate seed.

Mycotoxins in Scabby Grain

Several important mycotoxins can be produced in grain affected by scab. These toxins are produced by growth of the Fusarium fungi in the kennels. These mycotoxins can cause vomiting, nausea, dizziness, diarrhea, and muscle spasms in non-ruminant animals. Highly sensitive laboratory tests are currently used to detect mycotoxin contamination in grain. Therefore, the chances of toxic compounds getting into human food is almost nil. The problem arises when farm livestock are fed uncleaned scabby wheat, oats or barley. Three percent or more scabby kernels in feed may be poisonous to hogs. Hogs fed 10 percent or more scabby grain may vomit and refuse to eat the grain mixture. Cattle, sheep and mature poultry are much less susceptible to the mycotoxins. These toxins are quite stable and may remain in grain stored indefinitely.

Seedling Blight Phase of Scab

Before seed treatment fungicides were widely used, sowing scab-infected seed was a common cause of poor stands. Scabby kernels may be dead, or else germinate weakly. If the seedling manages to emerge from the soil, it frequently dies before it becomes established. Seedling blight is first noticed when infected plants appear light to reddish brown in color and may be covered with a mass of pink or whitish mold. If seedlings survive, they generally lack vigor, and frequently send up only a few tillers with small heads. A light to reddish brown root or crown rot may develop as the weakened plants mature.

Control for Head Blight

Figure 2. Shriveled, discolored kernels from a scab affected wheat head.

Scab affected wheat head.

1. There are few varieties of wheat, oats or barley highly resistant to scab, but in greenhouse tests some varieties restrict the development of the disease to one, or only a few, florets per head. In the field, some varieties appear more resistant than others because they flower earlier or later than other varieties, or because they shed their anthers more quickly than other varieties. These varieties look resistant because they have escaped infection by avoiding rains that supply free water on the surface of the heads for germination of the spores. Differences in susceptibility may also be due to physical barriers to infection of spikelets. Contact the OSU Extension office in your county for a listing of varieties with some degree of resistance to scab.

2. Plant cereals as far away as possible from old corn fields if stalk residues are left on the soil surface. No-till wheat seeded in old corn residues greatly increases the chance of scab. If conventional tillage is used, clean, deep plowing of all infested stubble and straw of cereals and weed grasses, corn stalks and rotted ears is recommended. Complete coverage of crop residues reduces head blight infection by reducing inoculum levels. Manure containing infested straw or corn stalks may harbor the fungus and should not be put on fields planted to small grains. When possible, plant wheat following a legume crop (soybean) and maintain a rotation with 2 to 3 years between wheat crops.

Control Seedling Blight

1. Sow only high quality seed that has been well cleaned to remove lightweight kernel. Seed treatment fungicides are available that will improve germination and control seedling blights when grain is used for seed. Obtain a copy of OSU Extension Bulletin 639, Seed Treatment for Agronomic Crops, for current seed treatment recommendations. Seed treatment will control seedling blight resulting from infested seed, but will not control head blight, foot rot or crown rot phases resulting from Fusarium infection.

2. Delay planting of winter cereals until the soil temperature is 60 degrees F (16 degrees C) or below to reduce chances of seedling blight. Planting should always be after the Hessian fly free date. Spring seeded grains should be sown as early as possible when soil temperatures are relatively cool.

3. Sow seed in a well prepared seed bed with adequate fertilizer to ensure good root growth and vigorous plant development.

4. Rotate small grains and corn with legumes, and leave at least one year between cereals, grass or corn before planting small grains.

Controlling Mycotoxin Contamination

1. When a high percentage of heads in a field are affected by scab, precautions should be taken to reduce mycotoxin contamination of the harvested grain. Mycotoxin contamination is usually highest in the more heavily diseased kernels. Adjusting the combine to blow out the smaller, shriveled kernels will help reduce mycotoxin levels.

2. Dry harvested grain to 13.5 percent moisture as soon as possible after harvesting. The Fusarium fungi cannot grow or produce mycotoxins in grain at this moisture level.

3. Store suspect grain by itself and do not mix with good quality grain. Mixing contaminated grain with good grain will only yield a poor product that may be difficult to sell.

4. Have suspect grain analyzed by a laboratory for contamination by mycotoxins, especially for deoxynivalenol (DON), before feeding to livestock.

Source:Ohio State Unviversity Extension, Plant Pathology
Head Blight or Scab of Small Grains,AC-4-96
Patrick E. Lipps

Apple scab is one of the most serious diseases of apple and ornamental crabapple. Disease development is favored by wet, cool weather that generally occurs in spring and early summer. Both leaves and fruit can be affected. Infected leaves may drop resulting in unsightly trees, with poor fruit production. This early defoliation may weaken trees and make them more susceptible to winter injury or other pests. Infected fruits are blemished and often severely deformed. Infected fruits may also drop early.

Symptoms

Symptoms first appear in the spring as spots (lesions) on the lower leaf surface, the side first exposed to fungal spores as buds open. At first, the lesions are usually small, velvety, olive green in color, and have unclear margins. On some crabapples, infections may be reddish in color. As they age, the infections become darker and more distinct in outline. Lesions may appear more numerous closer to the mid-vein of the leaf. If heavily infected, the leaf becomes distorted and drops early in the summer. Trees of highly susceptible varieties may be severely defoliated by mid to late summer.

Fruit symptoms are similar to those found on leaves. The margins of the spots, however, are more distinct on the fruit. The lesions darken with age and become black and "scabby." Scabs are unsightly, but are only skin deep. Badly scabbed fruit becomes deformed and may fall before reaching good size.

Causal Organism

Apple scab is caused by the fungus, Venturia inaequalis. It survives the winter in the previous year's diseased leaves that have fallen under the tree. In the spring, the fungus in old diseased leaves produces millions of spores.These spores are released into the air during rain periods in April, May and June. They are then carried by the wind to young leaves, flower parts and fruits. Once in contact with susceptible tissue, the spore germinates in a film of water and the fungus penetrates into the plant. Depending upon weather conditions, symptoms (lesions) will show up in 9 to 17 days.

The fungus produces a different kind of spore in these newly developed lesions. These spores are carried and spread by splashing rain to other leaves and fruits where new infections occur. The disease may continue to develop and spread throughout the summer. Because a film of water on leaves and fruit is required for infection to occur, apple scab is most severe during years with frequent spring rains.

Control

  1. The use of resistant or scab immune varieties is the ideal method for controlling scab. Currently there are several apple varieties that are totally resistant to scab. Backyard growers are strongly encouraged to consider using these resistant varieties in order to reduce or eliminate the need for fungicide applications around the home. Scab resistant apple varieties include: Prima, Priscilla, Sir Prize, Freedom, Liberty, Jonafree, Enterprise, Goldrush, Redfree, Pristine, Williams Pride, Novamac and Nova Easygro. All other varieties, including most commercially grown varieties are susceptible to scab; however, they differ in their degree of susceptibility. McIntosh, Cortland, Red Delicious and Rome Beauty are all very susceptible to scab. Golden Delicious and Jonathan are less susceptible. Lists of scab resistant ornamental crabapples are available from many nurseries and garden centers. Some of the many crabapples with both excellent scab resistance and superior horticultural characteristics for Ohio include: Anne E, Bob White, Molten Lava, Ormiston Roy, Prairifire, Red Jewel, Sargent, Sentinel, Strawberry Parfait and Sugar Tyme.
  2. Rake and destroy fallen leaves. This will reduce the number of spores that can start the disease cycle the next year.
  3. Where resistance to scab is not present, fungicide application is the primary method of control.

Rough patches on potatoes form dry media and alkaline media conditions.

This disease occurs as wart-like growths on the foliage and fruit of infected plants. On the tubers of potato plants, the scab can appear as either indented or raised areas of growth.

Crop rotation may not help prevent scab (agricultural Yearbook, 1997). Keep the pH level low. Too much alkalinity will only encourage the growth of scab. For organic gardeners this means not to use wood ashes where scab has been a problem. Sulfur is said to be an effective control also.

Scab is reduced in media with acidity below 5.2 pH (Agriculture Yearbook 1957). Use ammonium sulfate or sulfur to increase acidity of the media or soil. Wet media and spray with elder leaves. Can also be controlled by drying out growing area, reducing humidity.

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