Common Root Rot, Seedling Blight, Damping-Off

Cochliobolus sativus, Fusarium spp., Rhizoctonia spp. Pythium spp.

Biology

Common root rot is consistently one of the most damaging diseases of wheat and barley. It can also infect oats, rye and triticale. There are a number of phases to this disease complex; seedling blight, root rot and later in the season, a possible leaf spot phase. The seedling blight phase is less common on wheat than on barley. In the mature plant, root rot caused primarily by Cochliobolus sativus is the major cause of crop loss. The leaf spot and head infection phase is less important on the prairies than in the eastern provinces.

Initial infections originate from soil-borne spores. Seedlings become infected following germination and further infections continue throughout the growing season. Abundant spore production occurs on diseased tissue. Spores are spread by wind, water, cultivation and through infected seeds. Spores may remain viable in soil for several years until stimulated to grow by the presence of a host plant.

Cold soils, as well as drought and high temperature stress are important predisposing factors for an infection.

The strain of C. sativus that affects barley is different from the strain that affects wheat. Thus, the severity of root rot may be high in wheat and low in barley in a field where a root-rot susceptible wheat cultivar was grown continuously. The reverse is also true where barley was grown continuously. Thus a crop rotation, even between cereals, will reduce root rot problems.

Damage Description

Patchy emergence is usually the first indication that damage has occurred. Brown spots appear on the roots, subcrown internodes (that part of the plant between the seed and crown) and leaves. Infected plants may be stunted. Seedlings may die before or soon after emergence even though they only show slight damage.

Common root rot and crown damage lower yields. Infected plants tend to produce fewer tillers with smaller and fewer seeds per head. Some plants may ripen prematurely. Root rot in wheat and barley in Alberta causes average annual yield losses of 6 and 10 per cent, respectively.

Cereal crops can withstand a certain amount of seedling blight. The extra space, water, nutrients and light that would have been used by missing seedlings are taken up by neighboring plants to produce more tillers with larger heads and kernels compensating for the loss. No definite threshold levels are available.

Diagnosis

The presence and severity of C. sativus on common wheat can be determined by pulling up plants and examining the crowns and subcrown internodes for disease.

Collect 100 plants, composed of randomly selected groups of 5-10 plants each, and rate the disease level on the crown and subcrown internode by the following scale:

Clean (0) = no brown spots (lesions)
Slight (a) = up to 25% of area with lesions
Moderate (b) = 25-50% of area with lesions
Severe (c) = more than 50% of area with lesions

Use the formula (a+2b+4c) / 10, where a, b, c are the number of plants that are rated as slight, moderate and severe, respectively, in the 100 plants sampled. This gives an estimated yield loss as a percentage. Thus, if there were 25 plants in each of the slight, moderate and severe categories, the yield loss estimate equals 17.5%, calculated as follows:

25+2(25)+4(25) / 10 = 25+50+100 /10 = 175 / 10 = 17.5%

A modification of this method only uses two disease categories: clean to slight and moderate to severe. The percentage of plants in the second group is the rating. This multiplied by 0.4 equals the estimated percentages yield reduction. If 40 per cent of the plants are in the moderate to severe category, then the yield loss would equal 0.4 x 40% or 16%.

These methods may also be used to obtain a disease rating for barley, durum wheat and rye, but no specific data are available for guidance in
the conversion factor used to estimate yield loss.

Management Strategy

• Avoid continuous wheat or continuous barley.
• Use rotations with other cereal or non-cereal crops.
• Avoid deep seeding, which reduces plant vigor; a healthy vigorous plant is more likely to avoid or cope with root or crown infections.
• Apply adequate fertilizer especially phosphorus to promote root growth and reduce disease severity.
• Use quality seed that was grown the previous season.
• Turn under of stubble to help reduce infection levels.
• Use resistant varieties.

Seedling blight of barley caused by the common root rot fungus (Cochliobolus sativus)

Spot blotch of barley caused by common root not fungus - looks very similar to some types of net blotch.

Blotch point - kernel smudge in wheat. (Alternoria and Cochliobolus sp)

Common root rot on oats - a severe instance killing the entire plant.

Severe common root rot of wheat. The disease does not usually kill entire plants.

Healthy and common root rot diseased sub-crown internodes of wheat.

Spot blotch on wheat caused by Cochliobulus sativus the common root rot fungus.