Verticillium Wilt of Alfalfa

Agdex#:	121/600
Publication Date:	02/88
Order#:	88-036
Last Reviewed:	01/97
History:	Revision of Factsheet "Verticillium Wilt of Alfalfa" September 1986
Written by:	Harvey Wright - OMAF

Table of Contents

1. Introduction
2. Cause
3. Symptoms
4. Diagnosis
5. Disease Cycle
6. Control Measures -- When Disease is Present on the Farm
7. Control Measures -- Disease Not Present on the Farm

Introduction

Verticillium wilt of alfalfa is a disease which can cause serious stand thinning and yield loss in third year or older alfalfa fields. This disease has been prevalent in northern Europe since the early 1950's but did not become established in North America until 1976. By the late 1970's, verticillium wilt was widespread in Washington, Oregon and Idaho in the U.S., and in British Columbia in Canada. This disease was first discovered in Ontario in 1980. It is believed that the disease was introduced on contaminated seed. A 1983 survey found the disease in some 80 widely distributed fields, with the heaviest concentration west of Toronto and particularly in Perth, Oxford, Wellington and Waterloo Counties.

The disease has spread relatively slowly, except during the wet cool season of 1986, when there was a considerable increase across Southern Ontario in the number of infested fields. The disease is now fairly widespread in the Niagara Peninsula and is showing up on scattered farms throughout Eastern Ontario. In Northern Ontario, verticillium wilt has only been found so far on Manitoulin Island and in Temiskaming District.

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Cause

Verticillium wilt is caused by a fungus with many strains. Several crops, including potatoes, tomatoes, sunflowers, peanuts and alfalfa can be infected.

However, the strain which infects alfalfa is very specific to alfalfa, and does not affect these other crops. Likewise, the strains which affect these other crops do not infect alfalfa. The alfalfa strain may survive on the roots of some crop plants (clovers, trefoil, soybeans) and on some weeds (lambsquarters, mustard, plantain), but does not produce symptoms on these plants.

Verticillium wilt may infect alfalla plants through the roots, if the fungus is present in the soil. The plant can also be infected by spores through the cut stem during harvest. These spores may be carried by the cutter bar, on infected leaves, or by some insects. The infection progresses upward through the plant in the water conducting tissue, plugging these vessels and limiting water movement in the plant.

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Symptoms

Figure 1. Verticillium wilt symptoms in 4th year alfalfa stand.

Figure 2. Early leaf symptoms. Note light tan colouring.

Figure 3. Leaflets curling inward and upward and tan discolouration.

Figure 4. Typical wilting and discoloration caused by verticillium wilt.

Figure 5. Third year stand showing infected and healthy plants.

The first noticeable effect is a wilting of the leaves during the heat of the day, with plants recovering overnight. As the disease progresses, leaves show a pale yellow or light tan colouring, starting at the tips of the leaves (in a pattern similar to that caused by leafhoppers). Leaves will then curl upward and inward, continuing to lose colour until the entire leaf is a tan brown. Eventually these leaves shrivel up, but the plant stem will remain green and upright. A cross-section cut through the tap root of seriously infected plants will show a brown ring.

In the initial stages of disease, plants showing symptoms will have a normal growth rate compared to healthy plants. Often only one or two of the several stems from a crown will be infected. These plants will regrow after cutting, and appear healthy until regrowth reaches 15-20cm. Symptoms will then show up again, often severely stunting the growth rate. As normal plants continue to grow, the stunted diseased plants become hidden in the alfalfa canopy, making detection of affected plants difficult.

This is particularly true in the first cut growth. The disease is often easier to detect in the third cut and tall regrowth, where the shorter growth at that time of year does not hide the infected plants as much. Disease levels in an infected field are often highest in the tall regrowth, probably because disease spread is favored by the higher moisture and cooler temperatures that often occur during September.

It is rare to see symptoms of verticillium wilt in alfalfa during the direct seeding year, or the first production year after seeding under cereals. Most second year production fields show a low level of plants with symptoms. Occasionally, where the disease is well established on a farm and weather conditions are favorable to the disease, second year fields will be hurt significantly. It is during the third year that this disease can have a major impact, often affecting 20-30% or more of plants. Yield is reduced as plants die throughout the summer and over the winter, and weeds such as dandelions move in. Yields are often unprofitable in the fourth year, and the stand will continue to thin out as more plants die.

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Diagnosis

Discoloured leaves on alfalfa can be caused by potato leafhopper injury, by potash or boron deficiency, or even by severe drought. These problems are usually much worse in hot dry weather, and usually occur in patches in which almost all plants are affected. On the other hand, verticillium wilt shows up in both wet and dry weather and symptoms show on scattered plants more or less uniformly distributed throughout the field. Phytophthora root rot can also cause discoloured alfalfa leaves but it shows up mainly in the seeding year on heavier soils after a period of excessive rainfall. Assistance in diagnosing suspected verticillium wilt can be obtained through an OMAF Office.

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Disease Cycle

Verticillium wilt can be introduced to a farm through infested alfalfa seed. In this case, the disease is usually on small pieces of plant debris (such as pieces of seed pod) present in the alfalfa seed. It may also be present on the seed coat, or to a very limited extent, inside the seed. Infested seed is considered much less significant in spreading the disease now than are other factors such as equipment. The mower conditioner, forage harvester and baler can transport the disease between farms or between fields on a farm on infected alfalfa leaves or stems.

The disease can also be spread, either over distance or on the farm, by infested hay. Spores can live at least 9 months in hay. Unconsumed hay can end up with manure and be spread to a field. There is also evidence that the fungus will survive passage through the digestive system of sheep, although it did not survive in well-composted manure. Thus pasturing animals could spread the disease if fed infected hay. Verticillium wilt does not survive the ensiling process.

Evidence from Alberta has shown that the disease can also be spread (mainly within a field) by insects such as aphids, grasshoppers and leafcutter bees.

Unlike some soil borne diseases, verticillium wilt lives only a short time (probably not over 2 years) in soil when there are no roots of alfalfa or other host plants. But because there are several weed hosts (e.g. lambsquarters, mustard, plantain), weed control has to be very good to eliminate the disease from the soil by crop rotation.

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Control Measures -- When Disease is Present on the Farm

Resistant Varieties

The use of resistant varieties is the most effective way to control this disease. The number of alfalfa varieties with resistance is increasing rapidly. Refer to the list in the 1997 Ontario Forage Crop Variety Performance brochure.

Alfalfa varieties are currently rated as follows for their disease resistance to verticillium wilt:
S - susceptible (less than 15% completely resistant plants)
MR - moderately resistant (15% to 30% completely resistant plants)
R - resistant (31% to 50% completely resistant plants)
HR - highly resistant (more than 50% resistant plants)

There can be confusion over the interpretation of these resistance ratings. One could assume that if a variety has 30% resistant plants, the other 70% would be susceptible and killed if the disease were present. However, this is not true for verticillium wilt.

In any one alfalfa variety, there are a percent of plants with complete resistance, and a percent of plants highly susceptible. These susceptible plants would probably be killed if infected by the disease. However, there is also a significant percent of plants with partial resistance. A typical variety with a MR rating could have 30% resistant plants, 10% plants highly susceptible and 60% with partial resistance. Under field conditions, plants in this 60% group show considerable ability to resist the disease. Because of this, it is not necessary to have alfalfa varieties with 100% complete resistance in order to get effective control of the disease. It appears from current research that if 40% to 50% of the plants in a variety are completely resistant, this is adequate to prevent yield loss in the field. In the case of the similar alfalfa disease, bacterial wilt, a resistance level of 42% in the variety Vernal has given complete protection from that disease.

On farms where the disease is well established and affecting 20% or more of plants in third or fourth year fields, varieties rated MR will show much less disease than will varieties rated S. However, a certain number of plants, (varying with climatic and stress conditions), will probably be affected in the third or fourth production year. On these farms, varieties rated R would be preferred.

Harvest Procedures

Once the disease has been confirmed on a farm, one should attempt to limit its movement from older fields to newly established fields, if the new fields are planted to susceptible varieties. Verticillium wilt often shows up more severely near the field entrance, and in the first few rounds of a field, suggesting that harvest equipment spreads the disease readily. Consider harvesting younger nondiseased fields first. The mower conditioner, forage harvester or baler should be swept clean of leaves and stems after harvesting infected fields. Disinfect the cutter bar with a 10% bleach solution and rinse with water to prevent rust problems.

Crop Rotation

One can prevent reinfecting a new seeding of a susceptible alfalfa variety through the soil by keeping a diseased field out of alfalfa for two years before reseeding. However, this will not prevent infection from other diseased fields on the farm. Thus, the use of resistant varieties, particularly those with R ratings, is much preferable to rotation as a control method.

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Control Measures -- Disease Not Present on the Farm

Preventing Disease Introduction

Since 1986, alfalfa seed entering Ontario which has been certified as coming from a disease-free seed field, or tested and found free of disease, has not required seed treatment. Seed lots infected with verticillium require treatment with Thiram before sale. Thiram is effective in killing verticillium present in plant debris with the seed, or present on the seed. Introduction of the disease through seed is now considered an insignificant method of spread.

Introduction of the disease on custom harvesting equipment is a very real possibility, particularly if farms in your area have the disease. If possible, the sanitation methods discussed under "Harvest Procedures" should be used.

Verticillium wilt can also be introduced through purchase of infected hay, or may be carried by insects from nearby farms. These sources of infection are very difficult to control.

Variety Selection

On farms where disease is not present, it is not yet known whether planting of varieties rated MR would completely prevent verticillium becoming established. It would certainly reduce the level of disease.

Planting of varieties with a R level of resistance should prevent the disease from becoming established on the farm.

When the disease is not present on a farm or in the local area, there may be good reasons to continue to plant a variety which has performed well for you, but is susceptible. However, one should carefully check third and fourth year fields each year. If verticillium wilt does show up, varieties with resistance should be used in all new plantings. It may also be desirable to evaluate performance on your farm of one or two of the new high yielding verticillium resistant varieties even before the disease shows up.

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