SCLEROTINIA DISEASES
by Franklin Laemmlen
The fungus genus Sclerotinia (Sclair-o-tin-e-ah) contains a number
of important plant pathogens. Vegetable growers on the central coast are
probably most familiar with S. sclerotiorum (sclair-o-she-orum)
and S. minor, the causes of lettuce drop. Sclerotinia minor
attacks only lettuce. However, Sclerotinia sclerotiorum has a wide
host range which can include crops such as broccoli, cabbage, cauliflower,
carrots, celery, beans, tomato, peppers, potatoes, stocks, and sunflower.
Although seldom observed on the central coast, eggplant, squash, artichoke,
asparagus, beet, and broad bean can also be hosts to this pathogen along
with several flower crops and landscape shrubs.
The most obvious and typical initial symptom of S. sclerotiorum
or S. minor is the presence of a cottony, white, dense mat of mycelial
(mass of fungus strands) growth on the surface of the host and on the adjacent
soil surface. Whithin this fluffy white mass, dense white bodies of fungus
soon form. These bodies become black and hard as they mature and are called
sclerotia (pl) (sclair-o-she-ah) sclerotium (sclair-o-she-um) (singular).
The sclerotia act like seeds and allow the fungus to survive for several
years in the soil. S. sclerotiorum produces large (2-10 mm in diameter)
smooth, rounded sclerotia, while S. minor produces small (0.5-2
mm in diameter) rough, angular sclerotia. As the fungus colonizes host
tissues, it produces a pale brown to gray-brown lesion. Severe tissue degradation,
resulting in a mushy soft rot, also occurs. In celery, the white, cottony
growth and tissue rot are often accompanied by a pink/scarlet coloration
of the tissues. Hence the name "pink rot" for this disease in
celery.
Sclerotinia minor seldom produces spores. This pathogen usually
attacks the lettuce stem at or near the soil line. Lesions develop on the
stem, and the pathogen gradually "cuts" the plant in two at which
time the head collapses or "drops."
Sclerotinia sclerotiorum can attack its host through the soil similar
to S. minor, but it also frequently produces spores which can be
airborne to the host. Hence, it is not uncommon to see S. sclerotiorum
infections in the foliage of celery, on the heads of lettuce, cabbage,
broccoli or cauliflower, on bean pods, and the aerial parts of other hosts.
Senescent flower parts are an ideal site for this pathogen to colonize.
From this tissue the pathogen can quickly invade healthy leaves, stems
or fruits (pods). This is why S. sclerotiorum can be such a serious
pathogen in flowering crops, seed crops, and dry beans.
Both S. sclerotiorum and S. minor survive between crops as
sclerotia in soil or as mycelium in infected plant debris. Under central
coast conditions, sclerotia can probably survive up to 3 years in soil
in the absence of a host. Under dry conditions sclerotia have retained
their viability more than 10 years.
Control of Sclerotinia diseases must be accomplished by using a combination
of cultural and chemical means. To date resistant cultivars have not been
developed with sufficient resistance to make this a feasible means of control.
Activity of these pathogens is favored by high soil moisture and high air
humidity. Research has shown that the use of drip irrigation can dramatically
reduce both factors near the soil surface and thereby reduce the incidence
of Sclerotinia diseases. Crop rotation is another important tool in reducing
the sclerotial population in the soil.
Lettuce is highly susceptible to both S. sclerotiorum and S.
minor. Rotation with any other crop will help reduce S. minor,
but may not reduce the S. sclerotiorum population. S. sclerotiorum
can be reduced by planting corn, small grains or grass crops. It should
be mentioned that a non-crop fallow period does little to reduce the sclerotial
population. The wetting and drying of soil that occurs during a cropping
cycle is much more effective in reducing the number of active sclerotia
in the soil. Deep plowing has been recommended to help suppress Sclerotinia
diseases, but recent research has shed doubt on the usefulness of this
practice.
Finally, there are a number of fungicides, such as benomyl, chlorothalonil,
dichloran, iprodione, thiophanatemethyl, vinclozolin and metam-sodium,
that have excellent activity against Sclerotinia. Crops labelled for use
with each fungicide and use patterns vary. Consult the product label for
registered uses.
TIPS FOR SPRING VEGETABLE PRODUCTION
by Keith S. Mayberry, Imperial County
Vegetable transplant cells: the bigger the better? Yes, according to
Dr. Charles Vavrina, Associate Professor at the University of Florida,
who evaluated nine studies where 6 of the 9 showed significantly bigger
transplants at planting, and increased yield or greater total yield. When
growing high value crops, larger plant cells are worth the added cost.
The studies evaluated included crops such as tomatoes, watermelons, peppers,
muskmelons, cabbage, and broccoli. The phenomenon held true in studies
conducted in six states.
How deep should you plant bell peppers? A published study by Dr. Charles Vavrina indicates that transplants set to a depth of cotyledon leaves or to the first true leaf yielded more peppers than plants where just the rootball was covered. Studies were conducted both spring and fall. The researchers hypothesized that root temperature and a better ability to acquire water and nutrients may have been involved. Yield was improved by 30 percent in first harvest, but the overall yield was 13 percent better.
Does running sprinklers help with frost protection? Yes, however you must start the sprinklers much earlier than you think, depending upon the dew point (the temperature at which water vapor begins to condense). With a dew point of 28o F, the sprinklers should be started at 33o F air temperature. With a lower dew point of 21o F, the sprinklers should never be shut down until the ice is completely melted, even if the sun is shining. For more information, contact our office (805/934-6240) to get a two-page bulletin on frost protection via sprinklers.
Do cryoprotectant and antitranspirant chemicals work for frost protection? Scientific studies on several commercial products on tomatoes and peppers in North Carolina showed no beneficial effects. Similarly, another study on strawberries, using ice-nucleation-active bacteria to prevent freeze damage, indicated no benefit. A study at UC Desert Research and Extension Center on processing tomatoes similarly showed no benefit from ice-nucleation-active bacteria for frost protection.
What keeps eating my vegetable seeds and transplants in spring plantings?
The most likely cause of seed being hollow with no embryo is seedcorn maggot.
These widely distributed insects feed on decaying crop residues. Often
vegetables are planted into soils with high quantities of vegetable debris.
These soils can also be a hotbed of maggot activity. For a complete description
of the pest and control, contact our office at 805/934-6240.
Information adapted from AG BRIEFS, January 1997.
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