3 Scouting and Management of Diseases
A large number of diseases affect both yield and
quality of alfalfa. The primary diseases of alfalfa are fungal leafspots,
crown rot, sclerotinia stem rot, phytophthora root rot, and anthracnose.
Disease control in alfalfa depends heavily on use of resistant cultivars
and recommended agronomic practices to establish a vigorously growing crop.
Cutting or grazing rank growth will help lower the humidity in the foliage
and reduce the moisture that encourages pathogen growth and infection.
Areas that become heavily infested with soil-borne diseases may need to
be taken out of alfalfa production for two or three years to allow the
pathogens to die. When possible, rotate fields with com or sorghum to help
prevent the buildup of these pests. It is not economical to spray fungicides
for disease control in alfalfa.
Anthracnose
Anthracnose (Colletotrichum trifolit) symptoms
range from a few irregular, blackened areas on resistant stems to large,
sunken, oval to diamond-shaped lesions on stems of susceptible plants (Figure
1). The lesions are straw colored with brown borders. Black fruiting
structures (acervuli) develop in the bleached lesions and are readily visible
with a hand lens. The lesions enlarge, grow together, girdle, and kill
one or more stems on a plant. A conspicuous symptom of anthracnose is straw-colored
to pearly white dead shoots scattered through the field in summer and fall.
The crown rot phase of anthracnose is characterized
by bluish black discoloration of invaded tissue. This symptom is often
observed when killed stems are broken off at the crown. Sometimes lesions
cannot be found on the dead stems. If the stem is broken off and the base
is bluish black, the diagnosis is andiracnose. If the base has light brown
discoloration, the disease could be fusarium wilt or rhizoctonia crown
rot. These diseases may occur together in the same field. Other symptoms
of andiracnose include blackening and killing of petioles and occasional
formation of a "shepherds crook" when the stem dies suddenly while wilted.
Phytophthora Root Rot
Plants infected with phytophthora root rot (Phytophthora
megasperma) wilt, and the foliage, particularly the lower leaves, becomes
yellow. Regrowth of diseased plants is often slow after cutting. Lesions
with diffuse margins on the taproots are yellow to brown and usually start
where a lateral root emerges. The yellow discoloration of tissue that extends
through the root cortex into the xylem tissue is a diagnostic feature of
the disease. Taproots of numerous surviving plants in the field may be
rotted off at various depths., if conditions do not favor the pathogen,
new roots will form.
Sclerotinia Crown and Stem Rot
Leaves and stems infected with sclerotinia crown
and stem rot (Sclerotinia trifoliorum) become yellow and limp, and
finally collapse (Figure 2 and figure
3). White fluffy fungus growth covers dead plant parts on the soil
surface and initiates new infections. When the fungus exhausts its food
supply or when environmental conditions are unsuitable for its continued
growth, the fungus produces hard black sclerotia about the size of pencil
lead on or in stem and crown tissue. Sclerotia act as spores and can infect
new plants in subsequent years. Although sclerotia can live for many years,
they become less healthy and even die if there is no alfalfa to infect.
As a consequence, rotation to non-host crops can be a very beneficial control
technique. First year stands are usually hardest hit. Older stands are
less susceptible to this problem. Greatest damage occurs during early spring.
When conditions begin to warm up the disease usually becomes less severe.
Fungal Leafspots
Leafspots interfere with photosynthesis and cause
premature defoliation. The most serious leafspots in North Carolina are
Lepto leafspot, spring leafspot and summer leafspot. Rust occurs during
late summer and fan in the coastal plain and piedmont and can be severe
if harvest is delayed. Rusts can be controlled by planting resistant cultivars,
but cultivars resistant to other leaf spots are not available.
Spring black stem and leafspot (Phoma medicaginis)
may infect all above-ground parts of the plant, and the fungus may extend
to the crown and upper root. In early spring numerous small black-to-dark-brown
spots develop on the lower leaves, petioles, and stems. Young shoots are
often girdled and killed. Irregularly shaped lesions on leaves increase
in size, grow together, and become lighter brown; leaves turn yellow and
often wither before falling. Lesions on stems and petioles enlarge and
may girdle and blacken large areas near the base of the plant. The fungus
also causes a crown and root rot. In humid areas, seedpods may discolor
and shrivel.
Summer black stem and leafspot (Cercospora medicaginis)
produces small brown spots on both leaf surfaces. These enlarge to form
roughly circular, but indefinite, reddish brown to smoky brown lesions
0.1 to 0.2 inch in diameter. When relative humidity is at or near 100 percent
and environmental conditions favor spore production, the lesions become
ashy gray with silvery, glistening areas where spores are formed. Heavy
infections kill leaflets and cause severe defoliation. As the season progresses,
elongated, dark brown stem lesions enlarge and coalesce until most of the
stem is discolored. Small stems on nearly mature plants may die, resulting
in further defoliation.
Lepto leafspot (Leptosphaerulina briosiana)
primarily affects young leaves but also attacks petioles and other above-ground
parts (Figure 4). Leaf symptoms vary
with plant age, stage of growth, and environment. Lesions often start as
small black spots and either remain "pepper spots," as on white clover,
or enlarge to form oval or round "eye spots" 0.1 inch in diameter. The
lesions have light brown to tan centers and dark brown borders, often surrounded
by a bleached-out area. When conditions favoring infection and disease
development coincide with rapid regrowth, lesions appear as rather large,
light tan to almost white areas that grow together to kill the entire leaf.
High light intensity increases lesion size. The dead leaflets and petioles
often remain attached to die stem for a time. In older growth, the young
upper leaves become infected and show typical symptoms but seldom die before
harvesting.
Crown Rot (Rhizoctonia)
Although thizoctonia crown rot (Rhizoctonia solani
and other fungi) can cause root and stem rots, the disease is usually noticed
first as a crown rot. In bud rot and crown rot phases, brown lesions first
appear on the buds and young shoots below and at ground level. As the infection
progresses, buds and shoots die and the fungus grows into the crown, preventing
further production of new vegetative buds in the affected tissue. Sunken
whitish to brown lesions also develop near the base of a stem and may girdle
and kill it. Often brown bands develop in the older, lighter lesions that
indicate expansion of the infected areas. Coarse brown fungus growth may
be evident on the surface of the lesions. Rhizoctonia stem lesions may
be mistaken for anthracnose, but they do not have die black discoloration
(setae) common to anthracnose.
During long periods of hot, wet weather, particularly
in humid areas, R. solani attacks leaves and stems, which collapse
quickly to form water-soaked dead tissue. in a few days, if favorable weather
conditions continue, the fungus spreads to adjacent plants. A characteristic
symptom is dead leaves sticking to neighboring leaves and stems by the
strands of the fungus. Sometimes large areas of dead plants appear as though
killed by scalding water. While other fungi can and do cause crown rot,
Rhizoctonia is the most common fungus associated with this symptom.