Since the 1930s, lettuce anthracnose has been known to cause
damage to lettuce fields in western Washington. Historically,
the disease has not been very serious except when fields are
cropped continuously to lettuce. Urbanization, which limits availability
of land for crop rotation, has contributed to renewed severity
of the disease. In addition, increased acreage of the highly
susceptible Romaine lettuce types is being grown due to consumer
preference for this variety. The negative impact of the disease
on leaf appearance will likely remain a concern because most
lettuce grown in Washington is sold in supermarkets where quality
is of utmost importance.
Symptoms. The first symptoms
of anthracnose occur as small, circular watersoaked lesions appearing
on the lower leaves of affected plants. As the disease progresses,
spots multiply and enlarge, becoming more elliptical in shape
and turning straw-colored to reddish brown. Eventually lesions
on the midrib form pits or depressions and leaf lesions may fall
out, giving the leaf the characteristic "shot-hole"
effect (Figure 1).
Biology. Lettuce anthracnose,
or shot-hole, is caused by the fungal pathogen Microdochium
panattonianum. It infects all cultivated lettuce cultivars
as well as several Lactuca weed species including 'prickly lettuce'
(Lactuca serriola L.). The pathogen overwinters in plant
debris from transplant operations and diseased plants left in
seedbeds and production fields. Disease is initiated from asexual
spores called conidia or hardened masses of fungal mycelium called
microsclerotia.
Free water from rain, dew, or sprinkler irrigation is required
for spore dispersal, germination, and infection by this pathogen.
The optimum temperature for infection occurs at 68° to 72°F;
however, infection can occur over a range of 50° to 86°F.
The fungus infects plants through stomates or penetrates the
leaves directly. Spores can germinate and infect in as little
as 2-4 hours if there is continuous leaf wetness, but microsclerotia
require 4-6 hours under optimum conditions for germination and
infection. Spores are produced in lesions on leaves and are spread
to other leaves primarily by splashing water. Spores can travel
as far as 8 feet from a lesion source. Microsclerotia are formed
within infected leaf tissues and deposited on or in soil in lettuce
residues. It is unclear how lettuce plants become infected by
soilborne microsclerotia. Possibly, microsclerotia are deposited
on leaves by leaf contact with the soil surface or are splashed
directly onto leaves. They also may germinate on the soil surface
and produce conidia which splash onto the leaves (Figure 2).
Under ideal conditions, symptoms of the disease can appear in
seven days following infection. The conidia of this pathogen
are short-lived when exposed to drying, surviving only up to
9 days on dry lettuce leaves and 7 days on artificially contaminated
lettuce seeds. Microdochium panattonianum spores have
been reported to persist in artificially contaminated soil about
10 weeks. Microsclerotia, on the other hand, can survive in the
soil as long as 4 years.
Control. Disease control strategies
involve cultural and chemical control practices. Many of the
cultural controls now suggested are the same as those recommended
more than fifty years ago. Good weed control in lettuce fields
and adjoining land to remove prickly lettuce and other weeds
that can serve as hosts of this pathogen needs to be practiced.
Sanitation throughout transplant operations is critical. We suspect
that microsclerotia can be carried over in soil and plant residues
adhering to trays and benches. Therefore, greenhouse benches
and seedling trays should be thoroughly cleaned and then sanitized
using dilute bleach solutions to minimize possible disease transmission
from these sources. Cull piles and lettuce seedlings discarded
after transplanting should always be destroyed promptly. If allowed
to persist, they serve as a source for downy mildew and other
lettuce diseases in addition to anthracnose. Fields should not
be cultivated when plants are wet to reduce spread of disease.
Overhead irrigation can also contribute to disease spread and
should be timed to minimize periods of leaf wetness. Equipment
should be washed between fields to avoid transferring infested
soil from one field to another. Most importantly, crop rotation
to ensure that fields are rotated out of lettuce production for
4-5 years helps diminish soilborne microsclerotia, particularly
in fields with a history of severe anthrac-nose.
Fungicide treatments can provide partial protection and are
only effective if cultural control methods to reduce disease
pressure are practiced along with properly timed applications.
Lettuce plants must be treated with protectant fungicides before
infection and at regular intervals according to label directions.
See the Pacific Northwest Disease Control Handbook for
fungicides currently registered for control of this disease.
Although work is in progress to breed resistant cultivars,
no commercially satisfactory levels of resistance exist in commonly
grown cultivars. Recently, 24 cultivars of commercial lettuce
representing Romaine, Head, Bibb, Green Leaf, and Red Leaf types
were evaluated for resistance to anthracnose in western Washington.
The Romaine and Head lettuce types generally were the most susceptible
in this study while Green Leaf, Bibb, and Red Leaf types were
the least susceptible.
By Debra Ann Inglis, Plant Pathologist, and Michael L. Derie,
Agricultural Research Technologist, WSU-Mount Vernon; James A.
Kropf, Extension Agent, Pierce County Cooperative Extension
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