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Reference: Manitoba Agriculture, Food and Rural Initiatives
Plant diseases and deficiencies are a common cause of loss in forage legumes and grass production. Recognizing a disease or deficiency problem and selecting a workable solution is the best way to prevent or reduce the amount of crop loss.
The most common plant diseases and nutrient deficiencies found in Manitoba and
western Canada are examined for characteristics and control methods. Generally, plant diseases can be controlled by using resistant varieties, crop rotation, and in some
cases, fungicide application. Nutrient deficiencies are usually corrected by an application of the absent nutrient.
Black stem is caused by the fungus Phoma medicaginis and is the most common disease found in Manitoba alfalfa crops. The fungus may infect all above ground parts of the plant as well as the root and crown area of the plant causing crown rot and a general decline in vigor. Numerous small black to dark brown spots develop on the lower leaves, petioles and stems in early spring. These may expand into extensive black areas. Infected leaves under
severe attack turn yellow and fall prematurely. Large lesions also girdle the stem, causing the plant to wilt above the damaged area.
It is most often severe in periods of cool moist weather during the spring and fall. The fungus over-winters on diseased stubble and can be carried on the seed. Fortunately the fungus cannot exist in the soil for more than two years in the absence of alfalfa.
- Cut plants early before extensive premature yellowing reduces yields. Rotate with non-legume crops for several years before replanting alfalfa.
Common Leaf Spot
Common leaf spot, caused by the fungus Pseudopeziza trifolii, is a destructive foliar disease of alfalfa. This disease causes small circular brown to black spots on the leaflets. Close examination under a hand lens reveals a lighter brown raised disc, which is the fungus fruiting body or apothecia. Unlike spring black spot, the spots of common leaf spot remain distinct and usually do not blend together. Infected leaves turn yellow and drop prematurely. The fungus overwinters on plant debris. In the spring, spores are discharged and carried on the wind. Infection can occur throughout the summer, under moist conditions.
- Cut alfalfa before the leaves fall to reduce yield losses and hold down infection levels in future crops.
- Use Rambler alfalfa which is resistant to common leaf spot.
Downy mildew is a common disease in alfalfa, and occasionally sweet clover. Normal symptoms are a light green blotch on upper leaf surfaces, and a pale violet
downy growth of fungus mycelium and spores on the lower surface. The top part of the shoot often has a dwarfed, bunched appearance and infected leaves are twisted or rolled downwards. Cool, wet, humid weather favors this disease. Although the symptoms can be conspicuous, downy mildew does not usually cause severe damage to the plant. When conditions are favorable it often infests virtually every alfalfa field in an area.
- Use the cultivator Algonquin which is resistant to downy mildew.
- Growing cereals or grasses in the crop rotation in infected fields will eliminate overwintering inoculum. But wind borne spores from adjacent infected alfalfa fields can still cause infection.
Root and Crown Rot
Root and crown rot is caused by winter injury, a number of soil borne fungi or a combination of both. Winter injury generally occurs when there is inadequate snow cover either because of late cutting, so the stems are too short to trap snow, or a light snowfall.
Winter injury characteristically produces a brown discoloration of the internal crown tissue. Thus the damaged plant is open to infection by several fungi such as Fusarium spp. and Rhizoctonia solani which rot the crown and root. Infected plants are stunted, take on a chlorotic yellow or pale green color and lack vigor. Some die right away, others linger on doing poorly for several years. The damage is usually worst in old stands.
Winter crown rot or snow mold also kills alfalfa and other legumes. It is caused by a number of soil borne fungi active in late fall and early spring when the plants are dormant and the temperature of the soil around the crown of the plant is close to 0°C. Above 0°C these fungi are no longer active and the alfalfa is no longer susceptible to this disease.
Other fungi involved in this crown and root rot disease complex include Coprinus spp. (cottony snow mold), Plenodomus meliloti (brown root rot) a destructive disease in sweet clover and sometimes alfalfa in early spring, and Fusarium spp.
- Grow well adapted hardy cultivars.
- Maintain plant vigor with a high but balanced fertility level in the soil.
- Rotate with summer fallow or cereal crops for three years once an alfalfa field has been broken.
Crown Bud Rot
Rhizoctonia solani, Fusarium roseum and Phoma medicaginis.
Crown bud rot generally occurs in irrigated alfalfa or in moist soils. It destroys bud tissue on the crown and the upper root area. The disease usually becomes progressively worse with each passing week. Crown bud rot is most severe in fields suffering from winter injury, insect or mechanical damage.
- Maintain plant vigor by fertilizing according to soil test.
- Allow plants to recover in spring before cutting.
- Avoid cutting near the end of the season which depletes root reserves and promotes winter injury.
- Rotate with cereals or grasses for two to three years to reduce soil borne fungi which cause crown bud rot.
- Use resistant varieties Anchor, Beaver, Thor.
Verticillium wilt of alfalfa and sweet clover is caused by the fungus Verticillium albo-atrum. This European disease was introduced to North America in the 1970′s and has become a severe problem in southern British Columbia. It has also been detected recently in Alberta, Saskatchewan and southern Ontario fields. Verticilium wilt was detected in Manitoba alfalfa fields in 1998 but is not widespread.
Initial symptoms are a wilting of the upper leaves followed by a general wilting of the entire plant. Wilted leaves and stems turn white or yellow. The base of the stem may turn grey or black due to the presence of the fungus.
Verticillium wilt is spread by infected seed or by machinery carrying infected plant debris and spores from field to field. It’s more severe in irrigated fields. Infected stands die out within three years in British Columbia. The disease may build up at a much slower rate under Manitoba conditions, particularly in unirrigated fields.
- Alfalfa seed can be treated with the fungicide Thiram to control seed borne verticillium wilt.
- Plough down infected fields and rotate with cereals and grasses for at least two years. Avoid potatoes, sunflowers and sweet clover in the rotation as they are also susceptible to Verticillium wilt.
Northern Anthracnose is a disease of red clover caused by the fungus Kabatiella caulivora. Alsike and white clover are fairly resistant. An early symptom is the appearance of translucent, water soaked, spots on the stems and leaves. The lesions eventually enlarge and become dark brown with a lighter brown sunken center. Affected parts above the lesion wilt and die. Characteristically, infected stems and petioles curl over, resembling a shepherd’s crook. The plants do not die but the disease causes a loss of leaves, stems and flowers reducing hay or seed yields. The fungus overwinters on diseased stubble. In spring, masses of white or pinkish spores are produced in the old lesions, which are spread, mainly by splashing rain.
Northern anthracnose is more active in cool wet weather.
- Use the resistant cultivars – Norlac.
- Use cereals or grasses in crop rotation for two years.
Alsike and White Clover
Sooty blotch, caused by the fungus Cymadothea trifolii is a conspicuous disease in alsike and white clover. It is specially prevalent in cool moist fall weather. Dark brown to black lesions form on the underside of leaves, while the upper surface exhibits a pale green or chlorotic appearance. The leaves eventually shrivel and die. The Sooty Blotch can also prevent flowering. Infected foliage is reportedly toxic to livestock and can cause mouth ulcers.
- Rotate with crops other than clovers for three years.
Ergot, Claviceps purpurea, attacks almost all forage grasses but is found most frequently in: brome grass, wheat grasses and fescue grasses. Occasionally it is found on timothy and reed canary grass.
The characteristic symptom of the disease is the replacement of the normal seed with a black fungus body of the same size or larger than the kernel. This fungus body or sclerotia is very toxic to livestock.
- Cut grass crops intended for hay or silage before heading to prevent formation of the honeydew spores and subsequent sclerotia.
- Rotate with broadleaf crops. Cereals such as wheat, rye, barley and triticale are also susceptible to ergot. Oats are rarely affected and can be regarded as resistant. The ergot sclerotia are viable for only one year but remain toxic almost indefinitely.
Helminthosporium Leaf Spots and Blotches
Most forage grasses, especially brome and wheat grasses, are affected by Helminthosporium leaf spots and blotches. This group of fungi form dark brown lesions on leaves, stems, seeds and crowns of infected plants. It kills the leaves reducing hay yields. Weakened plants are also more prone to drought and winter injury.
- Avoid excessive use of fertilizer, particularly nitrogen.
- Avoid dense stands that impede air movement between plants so that leaves remain damp for long periods following a rain or dew.
- Rotate with non-grass crops for several years before replanting to grass forages.
Purple spot is the most common disease of timothy in Manitoba. The main symptom is the formation of small circular or oval purple to brown spots on the leaves with light grayish-brown centers. Severely affected plants may be stunted and the leaves dry out from the tip back.
- The disease is severest in sandy soils where plants are short of nitrogen, phosphorus and moisture.
- The varieties Bounty and Climax have intermediate resistance to purple spot. North American cultivars are generally more resistant than cultivars of European origin.
All forage grasses are subject to winter injury from a group of fungi collectively known as snow molds.
The more common offenders are: cottony snow mold (Coprinus spp.), pink snow mold or Fusarium patch (Gerlachia nivale), gray snow mold (Typhula spp.) and snow scald (Sclerotinia borealis).
The most common symptom is dead circular patches of grass in the spring, usually covered by cobweb-like fungal mycelium. Small black or brown sclerotia sometimes form on the leaf sheaths or on the mycelium mat. Pink snow mold usually occurs late in the fall with cool moist weather. It produces a pinkish-mycelium which attaches to plant leaves and stems. Other forms of snow mold appear when temperatures drop close to freezing. Snow mold is often noted where the snow has drifted and is slow to melt in the spring.
- Turf grasses can be treated in the fall with registered fungicides. Consult current provincial recommendations for updated list of registered fungicides.
Rust caused by the fungus Puccinia spp. attacks bluegrass. The disease appears as reddish brown rust pustules on the leaves which may turn chlorotic and die prematurely. Like powdery mildew rust is most often seen when the plants are short of nutrients, or mowed infrequently.
- Fertilize and water to maintain vigorous growth.
- Mow once a week.
- Rust is partially controlled by post-harvest burning in bluegrass seed crops.
- Most bluegrass cultivars have an intermediate level of resistance to rust. Merion bluegrass is susceptible.
Maximum growth of forage crops can only be achieved if an adequate supply of plant nutrients are present in the soil. Observation of visual symptoms is a useful method of identifying the cause of the nutrient disorder and the subsequent fertilizer requirements. Deficiency symptoms will differ depending on the species and the severity of the problem. Visual identification supplemented with chemical analysis of the soil are both necessary for determining the soil’s nutrient status and fertilizer amount required for optimum crop yields.
Plant tissue tests are also useful in providing specific information on the nutrient levels in the plants themselves.
Essential nutrients refer to those mineral elements which are required before a plant can complete its life cycle. Table 1 lists the 16 elements which are known to be essential to all crops. Macro elements include mineral elements required in large concentrations. Micro elements, required in smaller quantities, are equally essential.
Table 1. Essential Elements
Macro element deficiencies in forage crops are usually easy to observe. Micro nutrient deficiency symptoms are less common and more challenging to identify correctly. In fact, no such deficiencies have been reported for forage crops in Manitoba. Legumes are considered to be much more sensitive to micro nutrient deficiencies than the grasses. The tissue testing service available at the soil testing labs is particularly helpful in confirming whether deficiencies of micro elements are present.
Avoiding nutrient deficiencies is a more positive approach than trying to correct problems after occurrence. Deficiency symptoms often appear too slowly, with the damage done and yields suffering by the time they are recognized.
Nitrogen (N) Deficiency
If legumes have been inoculated with the proper strain of nodule bacteria, nitrogen fertilizer is not required. Legumes are capable of producing and maintaining their own nitrogen requirements through the process of nitrogen fixation. In the event that inoculation was not successful, nitrogen deficiency symptoms will be observed.
Nitrogen deficiency symptoms initially observed in older leaves are characterized by a general chlorosis or yellowing. A pink coloration of older petioles may be recognized in initial stages. As the symptoms progress older leaves will die and fall off, while younger leaves begin to yellow. Nitrogen deficiency reduces growth and branching in legumes.
If inoculation has not been successful, nitrogen deficiency can be corrected by applying inoculum to the established legume stand using a spray solution or by inoculating some inert substance such as cracked wheat. Such practices should be done during, or prior to, a rainfall.
Phosphorus (P) Deficiency
Legumes require relatively high amounts of phosphorus and many soils lack a sufficient quantity of it.
Phosphorus deficient plants appear stunted, with leaves turning abnormally dark green. Leaflets, which tend to fold together giving a pointed appearance, are often purplish in color on the undersides. Roots are light brown and restricted in growth. Stems will be reddish in color under severe deficiency.
Potassium (K) Deficiency
Potassium, although not needed to form plant parts, plays an essential role in many plant functions. Legumes remove large quantities of potassium from the soil, causing its supplies to become limited in continually cropped soils. A healthy potassium supply is more important for established stands and legume grass mixtures than seedling legumes. As legume yields are increased, potassium is removed in greater quantities. If adequate potassium amounts are not present, legume stands will quickly degenerate to grasses and weeds.
Potassium deficiency is perhaps the most easily recognized nutrient imbalance in legumes. Small white spots appear around the leaf margins. The tissue between the spots
then turns yellow and dies. Symptoms are more pronounced on the lower plant parts as potassium is lost to the younger leaves. Deficiency symptoms are usually most obvious on second and third cuts.
Calcium (Ca) Deficiency
Legumes are most sensitive to calcium deficiency under acid soil conditions. Soils with low pH levels should be fertilized (a mixture of calcium or calcium and magnesium compounds) with time for proper legume root growth and modulation growth. Legume seeds coated with lime and inoculum are recommended when soil is moderately acid.
Calcium deficiency symptoms are difficult to detect in the field. This deficiency can easily be avoided with a proper liming program. When calcium is limiting, the first symptom is collapse of the petioles of the youngest fully developed leaves. The undersides of the leaflets will
become reddish purple. Necrosis, the death of plant cells causing tissue darkening, will develop along the margins of the immature leaves followed by a curling of margins.
Magnesium (Mg) Deficiency
Magnesium, essential in the formation of chlorophyll, is important for both plant growth in animal dietary requirements. Magnesium also plays a role in nitrogen fixation and improves the utilization of phosphorus. Deficiencies of this element are more prevalent in acid, sandy soils particularly in wet seasons. Potassium, which tends to decrease magnesium uptake, is one of the most important factors in determining magnesium deficiency.
Deficiency symptoms include interveinal chlorosis with the older leaves being affected first. Margins of leaves may turn a reddish brown color. Deficiency problems can be corrected by liming with dolomitic limestone.
Sulfur (S) Deficiency
Legumes remove high amounts of sulfur from the soil. As atmospheric sulfur levels decline and the use of sulfur-free fertilizers increases, sulfur deficiencies in legumes, particularly alfalfa, are becoming more frequent. Sulfur deficiencies are most likely to occur in coarse textured, well drained soils. Studies in Manitoba have shown a dramatic response by legumes to sulfur.
Sulfur deficient legumes are characterized by a pale green to yellow discoloration of younger leaves and veins. Plants lacking sulfur will be small and slender with older leaves eventually turning yellow. Sources of sulfur include gypsum and ammonium thiosulfate (12-0-0-26).
Boron, like the other micro nutrients, is required in very small amounts. Although it is usually present in the soil, deficiencies occur most frequently on sandy high pH level or low organic matter soils. Since excess boron could be detrimental, the need must be established before applying this nutrient.
Alfalfa is one of the more sensitive crops to boron deficiency. Symptoms occur first in the younger plant parts, shortening internodes and turning upper leaves yellow and red, particularly on the leaf tips. Symptoms are most obvious just before flowering or during periods of dry weather. Boron deficiency symptoms in clover occur as red to purplish coloration of leaf tips and margins. Since boron deficiency symptoms can be confused with leaf hopper damage, potassium or magnesium deficiency, plant tissue samples should be taken to verify the problem. Boron deficiencies have not yet been reported in Manitoba.
Copper (Cu) Deficiency
Copper deficiency, common on peat and muck soils, has become increasingly important in Manitoba. Most copper deficiency problems are associated with the needs of the
animal consuming the forage rather than the plant itself. Although the plant may contain an adequate level of copper, it may also be high in molybdenum. When such a plant is consumed, the copper is tied up by molybdenum, resulting in a copper deficiency. This condition is known as molybdenumosis, since the real problem is an oversupply of molybdenum.
Copper deficiencies in legumes themselves may occur on weathered sandy soils, organic soils, and where soil pH is around 7-8. The first symptom of copper deficiency in legumes is the wilting of younger leaves. Leaves soon become a faded, greenish-grey color. Growth is reduced and internodes shortened so plant appears bushy.
Molybdenum’s importance to legumes is the essential role it plays in the process of nitrogen fixation. Molybdenum levels which are commonly deficient on acid soils can often be corrected by liming.
Since molybdenum deficiency is related to reduced nitrogen fixation, symptoms are similar to those of nitrogen deficiency. Legumes will turn pale green, exhibit reduced growth, and eventually lose lower leaves.
Iron deficiencies in legumes are very rare. Deficiencies are usually due to low availability of iron rather than lack of it. Iron may not be available in alkaline or calcareous soils.
Legumes do not appear to be sensitive to zinc since deficiencies are uncommon. Coarse textured soils which have been intensively farmed (usually under irrigation) may contain low levels of zinc. Zinc availability problems appear to occur on calcareous soils or where high rates of phosphorus have been applied.
Forage grasses are much less sensitive to many elemental deficiencies than forage legumes. Deficiency symptoms of the micro elements have not been reported in Manitoba. If micro element deficiencies are suspected, tissue samples should be submitted to a soil testing lab.
The most important difference between forage grasses and legumes is that of nitrogen fixation. Grasses are more sensitive to the nitrogen levels in the soil due to their lack of ability to fix atmospheric nitrogen. Nitrogen is usually the first limiting factor in grass production. Obtaining optimum yields and quality requires hay fields and pastures receiving fairly large amounts of nitrogen. Applications of nitrogen should be made first, before or during periods of active growth.
Forage grasses are frequently grown in mixtures of legumes. Under ideal conditions, legumes can supply considerable quantities of nitrogen for the grasses. Where conditions are less favorable for legume growth and nitrogen fixation, the amount of nitrogen available to the grass may be less than optimum.
Nitrogen deficiency symptoms in grasses are first observed on older leaves. Leaves will turn a pale green to yellow color. Nitrogen deficient grasses will respond rapidly to applications of nitrogen.
Phosphorus plays an important role in both plant and animal growth. It is one of the nutrients commonly deficient in Manitoba soils. Apply phosphorus if high quality forage is desired.
Phosphorus is especially critical for young seedlings; consequently, deficiencies are more common in seedling grasses than established stands. Deficiency symptoms in young plants are variable and not easily identified. Typical symptoms include slow regrowth accompanied by an abnormally dark green color, while stems and lower leaves acquire a red or purplish color.
Phosphorus deficiency in established stands is also difficult to define. In some phosphorus deficient grasses, leaf blade margins tend to curl upward resulting in blade rolling. Thin stands with coarse plants also characterize phosphorus deficiency. Since sparse stands can result from many other factors, diagnosis should not be based on this characteristic alone. Some forage grasses develop a more specific symptom where all leaves are uniformly dark green with purplish tinges occurring some distance back from the leaf tips.
Recovery from phosphorus deficiency following application is much slower than for nitrogen or potassium.
Potassium is usually not as deficient in Manitoba soils as phosphorus. But, it may become a limiting factor after soils have been cropped for some time. Potassium is required in large amounts by forage grasses, playing a vital role in many plant functions including disease resistance. Potassium supply is much more important in established stands than young seedlings. Potassium supply should be monitored throughout the growing period since supplies can be depleted within one season. Forage grasses tend to absorb much more potassium than actually needed if an excess is available. This process, known as luxury consumption is one reason it is impossible to build-up any reserve supplies of potassium. Potassium deficiency symptoms are first observed in older leaves which take on a yellowish streaked appearance. This is followed by scorching of leaf tips. Plants will appear stunted with short narrow leaves. Leaf diseases may also be prevalent when potassium is deficient.
Magnesium deficiency is not considered a problem for Manitoba forages. However, the importance of this nutrient to plants and animals warrants discussion of possible deficiency symptoms. Magnesium tends to be low on acid soils. It is added to the soil in limestone or as magnesium salts with some fertilizers. Forage grasses are frequently lower in magnesium than forage legumes. Low magnesium levels in forage grasses may result in a nutritional disease in livestock called grass tetany. This disease is caused by an imbalance of the ratio of potassium to the total amount of calcium plus magnesium [ K / (Ca + Mg)].
The appearance of bleached or pale yellow leaves in grasses indicates the occurrence of potassium deficiency. In sod farming grasses this occurs as streaks of stunted grass with varying amounts of leaves showing bleaching. Winter injury is aggravated by magnesium deficiency. Soluble magnesium salts or dolomitic limestone can be added to correct magnesium deficiency problems.
Calcium appears to be present in sufficient quantities in most Manitoba soils. Calcium is added to soils in limestone, superphosphate and other fertilizer materials. Response to limestone applications by grasses may in part be due to the addition of calcium. Although calcium deficiency has not been reported in forage grasses, it is consumed in relatively large amounts by grasses. Calcium plays a vital role to the animal consuming the plant.
Sulfur plays an important role in the growth of forage grasses. Although forage grasses absorb small quantities of sulfur as compared to nitrogen, supplies may be limiting in soils low in organic matter. In grass-legume mixtures, grasses appear to be more efficient at absorbing sulfur. Low sulfur supplies may result in grasses taking over the stand completely, and signs of nitrogen deficiency becoming evident.
Severe sulfur deficiency symptoms are observed as a chlorosis of younger leaves. The older leaves will remain alive for some time which is in contrast to the symptoms of nitrogen deficiency. Sulfur deficiency can be corrected quickly by the application of sulfur containing materials. Calcium sulfate, elemental sulfur and sulfur bearing sources of nitrogen, phosphorus, and potassium are available.