Verticillium Wilt of Alfalfa
Table of Contents
- Introduction
- Cause
- Symptoms
- Diagnosis
- Disease Cycle
- Control Measures -- When Disease is Present on the Farm
- 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.
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.
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.
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.
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.
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.
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.
