Figure 1 |
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Figure 2 |
Figure 3 |
Host Crops
Sunflowers, canola, mustard, dry beans, field peas, lentils, potatoes
Biology
Sclerotinia survives in the soil for up to four years as irregularly-shaped fungal bodies
called sclerotia. The sclerotia germinate in the summer, producing mushroom-like
structures that release wind-borne spores. The disease is favoured by extended periods of
rain, several weeks prior to and during flowering.
The symptoms of sclerotinia wilt usually appear during flowering. The diseased plants
may occur singly or in patches. The upper leaves droop, dry out and die. The entire plant
dies quickly. Young wilted plants often turn black whereas mature plants usually remain
light brown. Small irregularly-shaped bodies can be found inside the stem near the soil
line. Surviving plants may or may not produce seed, depending on when the infection
occurs. The heads on wilted plants generally are smaller than those on healthy plants and
seed weights are lower.
Head rot (Figure 3) is caused by wind-borne spores of the same fungus. It usually occurs late in
the season, after initial flowering and extended periods of wet weather. Dead petals, on
the backs of the heads, provide a place for the spores to germinate. The affected heads
decay, become soft, and black sclerotia form within the heads.
Head rot also causes a decrease in oil content and an increase in free fatty acid
content. The sclerotia that form in diseased stalks and heads are returned to the soil at
harvest and can cause sclerotinia diseases in sunflower or other susceptible crops in
following years.
Symptoms of Damage
In general, the symptoms of damage include drooping, brown or dead upper leaves; plants
that turn brown or black and die prematurely (Figure 1); fungal bodies (sclerotia) in the
stem at the soil line; (Figure 2) soft, rotting heads (Figure 3) with only fibrous tissue
remaining; and black sclerotia in the heads. The characteristic symptoms of sclerotinia wilt include sudden wilting of leaves, root
rot, and a basal stem canker. The first wilted plants are usually seen just before
flowering but most appear in patches during and after flowering. The time required from
incipient wilt to complete wilting may be four to seven days.
The first symptoms of head rot usually are the appearance of water-soaked spots or
bleached areas on receptacles (the fleshy back of the head). The fungus can decay the
entire receptacle and the seed layer falls away leaving only a bleached, shredded skeleton
interspersed with large sclerotia.
Scouting Techniques
With sclerotinia wilt, young wilted plants often turn black. Mature plants usually
remain light brown. Small, irregularly-shaped, sclerotia can be found inside the stem,
near the soil line.
The bleached, skeletonized heads associated with head rot are very obvious in the
field, even from a distance. Infected heads usually shatter during combining and any
remaining seeds are lost. Usually, the seeds are not decayed but many are empty. The large
sclerotia in the heads may be 12 millimeters (1/2 inch) or greater in diameter and many are
harvested along with the seed. Large sclerotia mixed in with seed confirms that a field
contained head rot.
Economic Thresholds
Fungicides are not effective in controlling sclerotinia rot. Wilt occurs whenever
sunflower is planted on sclerotinia-infested soil and can cause severe yield loss. On the
average, infected plants yield less than 50 per cent of healthy plants. Sclerotinia wilt also
leads to increased levels of sclerotia in the soil which, in turn, can result in the
removal of fields from sunflower production for many years.
Control Tips
The key to managing these diseases is to plant in non-infested soil and prevent the
buildup of sclerotia in soil.
Monitor fields for sclerotinia diseases and rotate crops. Follow a crop rotation of 5
years between susceptible crops.
To minimize head rot, plant to maximize air circulation. Locate new plantings as far
away as possible from fields where sclerotinia was know to be a problem in the previous
four
years.
Although no resistant hybrids are available, progress is being made in reducing the
susceptibility of sunflowers to sclerotinia. |
