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Alfalfa Disease Management


INTRODUCTION

Alfalfa, Medicago sativa spp. sativa L., originated in the region of Asia Minor, Transcaucasia, Iran, and the highlands of Turkmenistan, where it has been cultivated since before recorded history (9). The oldest known reference to alfalfa is from Turkey (1300 BC) 3,300 years ago. The word "alfalfa” is Arabic meaning "good fodder.” Leaves and stems of alfalfa are high in protein and serve as an excellent and nutritious feed for many domesticated animals.
From its point of origin, alfalfa has been spread by man to the far reaches of the globe. It was first successfully grown in the western U.S. about 1850 and is now grown in every state in the continental U.S. It is believed to have been introduced into the Wyoming Territory in the late 1800s. Today alfalfa is grown in all 23 counties within Wyoming. There are currently an estimated 620,000 acres of alfalfa grown for hay production, of which 440,000 acres are irrigated and 180,000 acres are grown under natural rainfall (dryland) conditions. Additionally, an estimated 8,000 acres of alfalfa, most of which are located in the Wind and Big Horn River Valleys, are grown for seed production.
Although forage production is greater under irrigation than under dryland (2.3 tons/acre versus 0.8 tons/acre), diseases are more numerous and damaging in irrigated fields. Since alfalfa seed fields are not irrigated as frequently as fields grown for forage, diseases are not only fewer but are also less severe.
Like most crops, alfalfa is attacked by many disease-causing organisms. Seedlings as well as seeds, stems, leaves, and roots of older plants all serve as food sources for a number of disease-causing organisms. Of the 80 different diseases that have been reported on alfalfa from North America (6), 16 have been identified in Wyoming. Other diseases may occur in the state but have not been detected. Crown rots, root rots, and wilts, which can cause plant death, are usually worse than leaf and stem diseases. Under certain conditions, seedling diseases can damage newly planted stands. The presence and severity of plant diseases are largely dependent on the cultivar planted and the environmental conditions present. Loss from leaf and stem diseases is usually worse in years having higher rainfall as well as in alfalfa grown under irrigation. Foliage diseases are usually worse in sprinkle-irrigated fields while loss from root and crown diseases is usually worse where alfalfa is grown under furrow irrigation.

DISEASE IDENTIFICATION
Information on the 16 alfalfa diseases identified in Wyoming is presented in Table 1. Diseases are listed in alphabetical order. Age and plant part attacked, characteristic symptoms, distribution within the state, and suggested control practices are given.
One or more colored photographs is provided for each disease. After reading Table 1, see if you can recognize any of these diseases. If you are not sure what diseases are present in your field, contact your University of Wyoming agriculture extension educator for instructions on submitting a diseased plant sample for positive identification.

Stand Decline Diseases
In Wyoming the most damaging diseases of irrigated alfalfa, in order of importance, are Verticillium wilt, Alfalfa stem nematode, Phytophthora root rot, and Brown root rot (Disease No. 10, 14, 5, 2, Table 1). Both Alfalfa stem nematode and Brown root rot reduce stored carbohydrates and proteins in plant crowns and upper roots that are needed for winter survival and predispose plants to low-temperature injury and winterkill. All four of these diseases can cause a reduction in the plant stand and yield of susceptible varieties. Verticillium wilt results in the plugging of waterconducting vessels and reduced movement of water and essential nutrients within a plant. Plants with Verticillium wilt usually die during the summer months. Alfalfa stem nematodes parasitize new stem buds prior to their emergence from the soil, resulting in severe swelling and stunting.
Studies recently conducted by University of Wyoming researchers found the Chrysanthemum foliar nematode Aphelenchoides ritzemabosi as a co-habitant with Alfalfa stem nematode (Ditylenchus dipsaci) in diseased alfalfa stem and bud tissue. Although both of these nematodes may be present in alfalfa fields throughout the western U.S., reference will only be made to Alfalfa stem nematode for the purpose of brevity. Severely parasitized plants usually die during the winter months. Phytophthora attacks and kills seedlings and also causes root rot in older plants. Diseased plants may die anytime during the growing season or during the winter months. Brown root rot is a new disease in Wyoming, identified near Farson in Sweetwater County (elevation 6,800 feet) during 1996. This disease has been found to occur at most high elevation growing areas in the state where alfalfa is grown. It is associated with the periodic severe winterkill of plants. Additional information on these four important diseases are available (1, 4, 5, 8).

Table 1. Diseases of alfalfa in Wyoming.
Diseases of alfalfa in Wyoming.
Diseases of alfalfa in Wyoming.
Diseases of alfalfa in Wyoming.
Diseases of alfalfa in Wyoming.
Diseases of alfalfa in Wyoming.

Verticillium wilt

Verticillium wilt


Alfalfa stem nematode

Alfalfa stem nematode


Brown root rot

Brown root rot


Phytophthora root rot

Phytophthora root rot


Northern root-knot nematode

Northern root-knot nematode



Foliage Diseases
Several foliar (leaf and stem) diseases occur in Wyoming. The most prevalent are Common leaf spot, Downy mildew, and Spring black stem and leaf spot (Disease Numbers 3, 4, 7, Table 1). Less frequent foliar diseases include Anthracnose, Stemphylium leaf spot, and Summer black stem and leaf spot (Disease Numbers 1, 8, 9, Table 1). Of these diseases the most important are Common leaf spot and Spring black stem and leaf spot, which can reduce yield and forage value. Alfalfa plots containing standard check varieties were established at six locations in Wyoming and foliage diseases were assessed over a three-year period (1980-1982). Five locations were irrigated (Afton, Laramie, Powell, Riverton, and Torrington) while one (Sheridan) was grown under dryland conditions. Plots were visited twice yearly and foliage diseases identified and varieties rated for disease severity.
Results are presented in Figure 1. Spring black stem was the most prevalent disease at all six locations while Downy mildew and Common leaf spot occurred at three locations each. Yellow leaf blotch was identified at the dryland site only. Yellow leaf blotch (Disease Number 11, Table 1) appears to be the only foliage disease of importance in dryland alfalfa fields in Wyoming. This disease may cause severe leaf defoliation during certain years.
Bacterial stem blight (Disease Number 12, Table 1) occurs after spring frosts, resulting in severe damage to first-cut yields.
This disease has been identified in irrigated fields on several occasions in Sheridan and Buffalo counties near the foothills of the Big Horn Mountains. It may occur elsewhere in the state at higher elevations.
Alfalfa mosaic virus is a seed-borne virus disease which becomes systemic throughout the plant (Disease Number 16, Table 1). Damage from this disease is considered minor.

Severity of four foliage diseases of alfalfa at six locations in Wyoming. Disease ratings were based on a scale of 1-5 in order of increasing disease severity (1 = no disease, 2 = slight, 3 = moderate, 4 = severe, 5 = very severe). Values are the highest disease rating given a cultivar at the site during the three-year period from 1980 through 1982. All test sites were irrigated except Sheridan, which was dryland.
Figure 1. Severity of four foliage diseases of alfalfa at six locations in Wyoming. Disease ratings were based on a scale of 1-5 in order of increasing disease severity (1 = no disease, 2 = slight, 3 = moderate, 4 = severe, 5 = very severe). Values are the highest disease rating given a cultivar at the site during the three-year period from 1980 through 1982. All test sites were irrigated except Sheridan, which was dryland.


Common leaf spot

Common leaf spot


Downy mildew

Downy mildew


Spring black stem and leaf spot

Spring black stem and leaf spot


Anthracnose

Anthracnose


Stemphylium leaf spot & Summer black stem and leaf spot

Stemphylium leaf spot & Summer black stem and leaf spot


Sclerotinia crown and stem rot

Sclerotinia crown and stem rot


Yellow leaf blotch


Yellow leaf blotch


Bacterial stem blight

Bacterial stem blight


Alfalfa mosiac virus

Alfalfa mosiac virus



DISEASE MANAGEMENT
Management practices of irrigated alfalfa, aimed at reducing loss from plant disease, are based on factors such as location in one of ten disease zones in the state, selection of an appropriate multiple disease-resistant variety, and use of other disease management practices including environment modification, crop rotation, irrigation, and fungicides.

Disease Zones
To assist in the management of alfalfa diseases, a state/county map was developed with ten separate disease zones (Figure 2). Disease zones were designed on the basis of geography, historical laboratory, and field disease records. Diseases which have been identified in each zone are listed in order of importance (Table 2). Additional information on when or under what conditions specific diseases may be expected to be severe is given under the "Comments” section. Knowledge of what diseases occur in your zone and which are most important is critical for selecting the best diseaseresistant variety.

Ten Alfalfa Disease Zones in Wyoming and Foliage Disease Nursery Sites
Figure 2. Ten Alfalfa Disease Zones in Wyoming and Foliage Disease Nursery Sites (•).


Selecting Multiple Disease-Resistant Varieties. Currently there are 252 certified varieties of alfalfa available for purchase in the U.S. (7). Of these, 167 with fall dormancy ratings of 2-4 can be adapted to the growing conditions in Wyoming. Names of these 167 varieties, marketing information, and disease ratings for the four major stand-decline diseases which occur in Wyoming are given in Table 3. Diseases include Verticillium wilt (VW), Phytophthora root rot (PRR), Stem nematode (SN) and Brown root rot (BRR). Alfalfa variety reaction to the Northern root-knot nematode (NRKN) is also provided for alfalfa growers in eastern Wyoming. Disease ratings are explained in the footnotes at the bottom of each page. Varieties are also rated for fall dormancy (FD) on a scale of 1-10; 1 equals most fall dormant or winterhardy, and 10 equals least fall dormant or winterhardy. Currently there are no FD 1 varieties available or recommended for the U.S. In Wyoming, varieties should be chosen with FD ratings of 2-4, with 2s and 3s being grown at higher elevations and 3s and 4s grown at lower elevations. Varieties with FD ratings of 2 and 3 will go dormant sooner than varieties with higher FD ratings and will survive the harsh Wyoming winter.
When selecting a variety, emphasis should be placed primarily on the diseases which occur within a given zone that are known to cause stand and yield decline: Verticillium wilt, Stem nematode, Phytophthora root rot, and Brown root rot. Unfortunately, there are no certified varieties developed in the U.S. that have known resistance to Brown root rot. The U.S. variety Ranger (FD=3) and the Canadian variety Peace (FD=1) have shown field resistance to Brown root rot in Sweetwater County. To assist in the selection process, varieties with a "resistant” (R) rating or greater for Verticillium wilt, Phytophthora root rot, and Alfalfa stem nematode in Table 3 have a single asterisk while those with a "highly resistant” (HR) rating to all three diseases have double asterisks.
Information on resistance to foliage diseases is not readily available. Varieties with resistance to Common leaf spot and Downy mildew are available. Marketers and/or alfalfa seed companies can provide information on foliage disease resistance for their varieties. Varieties with resistance to Bacterial stem blight have not been developed, and there are currently no varieties with resistance to Yellow leaf blotch. If it is not known what diseases are present, select a variety based on diseases given for the disease zone in Table 2. For an example of variety selection, let’s say a producer lives in Goshen County within Disease Zone I shown in Figure 2. Ten diseases have been identified in this zone and are listed in Table 2. 
Diseases are listed in order of importance. Verticillium wilt is the worst disease in Goshen County. Select a variety with an R or HR rating to Verticillium wilt, using an HR rating if the field will be irrigated by sprinkler. Care should be taken to consider soil type (sandy versus clay loam) and type of irrigation (furrow versus sprinkler) which may increase certain diseases. This is especially true for Stem nematode and Phytophthora root rot, which are generally more severe in fields with a high clay content (river bottom land) and Northern root-knot nematode, which is more severe in fields with a sandy loam soil. All will be worse under furrow irrigation. Several varieties are available with R or HR rating to all three of these diseases (Table 3). After selecting one or more varieties, check with a seed distributor or marketer on possible resistance to Common leaf spot.
Optimal yields and extended stand persistence should be obtained by selecting a variety with as high a level of disease resistance as possible. Check with a University of Wyoming agriculture extension educator to determine which of the varieties selected have been tested in Wyoming (3).

Table 2. Principal Alfalfa Diseases in the Ten Disease Zones in Wyoming.
Principal Alfalfa Diseases in the Ten Disease Zones in Wyoming
Principal Alfalfa Diseases in the Ten Disease Zones in Wyoming
Principal Alfalfa Diseases in the Ten Disease Zones in Wyoming

Table 3. Disease Ratings of 2003-2004 Certified Alfalfa Varieties adapted to Wyoming with Fall Dormancy (FD) Ratings of 2 to 4.
Disease Ratings of 2003-2004 Certified Alfalfa Varieties adapted to Wyoming with Fall Dormancy (FD) Ratings of 2 to 4
Disease Ratings of 2003-2004 Certified Alfalfa Varieties adapted to Wyoming with Fall Dormancy (FD) Ratings of 2 to 4
Disease Ratings of 2003-2004 Certified Alfalfa Varieties adapted to Wyoming with Fall Dormancy (FD) Ratings of 2 to 4
Disease Ratings of 2003-2004 Certified Alfalfa Varieties adapted to Wyoming with Fall Dormancy (FD) Ratings of 2 to 4
Disease Ratings of 2003-2004 Certified Alfalfa Varieties adapted to Wyoming with Fall Dormancy (FD) Ratings of 2 to 4



Other Disease Management Practices
Environment Modification
Plant disease severity is usually increased if air and soil temperatures are favorable and prolonged periods of optimum moisture occur. Phytophthora root rot and Alfalfa stem nematode both require water-saturated soils for plant infection to occur. Stem nematode infects newly forming stem buds which emerge from the crown just below the soil surface. Phytophthora infects roots several inches below the soil surface. As previously mentioned, the diseases are worse in flood-irrigated fields with heavy soils and in low areas of fields, both of which retain moisture longer. Managing irrigation to avoid prolonged periods of soil saturation and waiting until regrowth is 6 to 8 inches high before irrigating will reduce the severity of both diseases. Leveling fields prior to establishing alfalfa, particularly fields with heavy soil, will also aid in managing Phytophthora root rot and Stem nematode. Air-borne spores of Verticillium wilt can infect plants through recently cut stems. This usually occurs immediately after swathing, especially if a field receives rain or is sprinkle irrigated. A higher incidence of Verticillium wilt occurs in sprinkler-irrigated fields. Swathing fields when foliage is completely dry and waiting for two to three days before irrigating should reduce the spread and damage related to Verticillium wilt.

Crop Rotation
Rotating alfalfa fields with small grains or corn for two to five years will reduce pathogen populations and increase the performance of the disease-resistant variety of alfalfa when alfalfa is replanted. The longer the rotation, the better the control. Some diseases are reduced more than others depending on the presence of long-term survival structures. Crop rotation, although helpful, has less effect on the incidence of Brown root rot and Phytophthora root rot than other diseases present in Wyoming.

Fungicides
Seedling diseases may occur in fields when environmental conditions are optimum for their development (cool, wet soils). This is particularly true in fields where Phytophthora root rot is present. Severe seedling disease in alfalfa has been observed in Wyoming in new seedings in fields with a history of growing alfalfa and of having a high clay content (25 percent or greater) and in old alfalfa stands that are interseeded, a practice that is not recommended. In both cases disease damage is usually worse during cool, wet periods which tend to occur in the spring. In new seedings the use of the fungicide metalaxyl as a seed or soil treatment, even with resistant varieties, should reduce seedling loss when environmental conditions are favorable for disease development. Metalaxyl seed treatment will also provide protection for seedling alfalfa from downy mildew.
Although sulfur and copper fungicides can be used to reduce foliar diseases in alfalfa, few are applied in Wyoming. However, under severe foliage disease pressure, particularly when Spring black stem and/or Common leaf spot are present, either of these fungicides should reduce leaf defoliation. Application of fungicides should be made before a disease becomes severe and should be applied with a ground sprayer if possible. Bi-weekly applications of copper hydroxide provided moderate control of Spring black stem in southeastern Wyoming and significantly increased forage yield at the first harvest.

INTEGRATED DISEASE MANAGEMENT
Managing Old Stands
• Avoid excessive irrigation, especially in fields with high clay soils, since this will increase Alfalfa stem nematode and Phytophthora root rot. Fields that are flood irrigated are more likely to be over irrigated than those that are sprinkler irrigated. Remember, Verticillium wilt is usually worse in sprinkler-irrigated fields.

• After harvesting fields that are flood irrigated, allow 6 to 8 inches of regrowth before irrigating. This practice will reduce infection by Stem nematode of young stem buds which initiate below the soil surface.

• Fields known to have Verticillium wilt should be harvested last. Before reusing a swather, pressure wash it to remove all alfalfa plant material and allow it to dry.

• Interseeding alfalfa into old stands which are declining from disease is not recommended. This is particularly true if Alfalfa stem nematode and/or Phytophthora root rot are present since seedlings are particularly susceptible to both diseases.


Establishing New Stands
• Choose a certified variety with as high a level of resistance as possible to diseases present in the disease zone. Plant varieties with a fall dormancy (FD) rating of 2 or 3 at higher elevations and 3 or 4 at lower elevations. Use all information available on variety yield performance from university tests conducted in Wyoming (3), neighboring states, and by other growers to further aid in the selection of the best variety.

• Plant new stands in fields after a two to three-year rotation with other crops. This will allow time for a reduction of most alfalfa disease pathogens.

• If fields are furrow irrigated, level them prior to seeding to allow for good drainage.

• If Phytophthora is known to be present, select a variety with an HR rating. Metalaxyl, applied as a seed or soil treatment, should provide added protection of alfalfa seedlings during establishment.

• Add Rhizobium inoculant in the seed box prior to planting if seeds are not already pre-inoculated. This will allow for the formation of root nodules for a maximum fixation of atmospheric nitrogen.


OTHER RELATED PUBLICATIONS
1. Alfalfa Analyst. Revised edition. Alfalfa Council. 10920 Ambassador Drive, Suite 302, Kansas City, MO 61453. Published yearly.

2. Alfalfa Stem Nematode Biology and Management. 1993. University of Wyoming, Agricultural Experiment Station Bulletin No. B-761R.

3. Guide for Selecting Alfalfa Varieties for Irrigated Stands in Wyoming. 1995. University of Wyoming, Agricultural Experiment Station Bulletin No. B-1009.

4. Biology and Management of Phytophthora Root Rot of Alfalfa. 1996. University of Wyoming, Agricultural Experiment Station Bulletin No. B-791R.

5. Brown Root Rot of Alfalfa. 1999. University of Wyoming, Department of Plant Sciences, College of Agriculture. Plant Science Timely Information Series. Bulletin No. 1.

6. Compendium of Alfalfa Diseases. 1990. The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. 2nd Edition.

7. Fall Dormancy and Pest Resistance Ratings for Alfalfa Varieties 2001/2002 Edition. Alfalfa Council. 10920 Ambassador Drive, Suite 302, Kansas City, MO 61453. Published yearly.

8. Verticillium Wilt of Alfalfa. 2001. University of Wyoming, Agricultural Experiment Station Bulletin No. B-1022R.

9. World Distribution and Historical Development. 1988. Chp. 2. Alfalfa and Alfalfa Improvement. Agronomy Series No. 29.

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