More on take-all of wheat

CATHY DE VILLIERS, ARC-SMALL GRAINS INSTITUTE, AN INSTITUTE OF THE ARC FIELD CROPS DIVISION, BETHLEHEM

Root diseases are typical of diseases that occur under irrigation practices – or areas with high rainfall and low temperatures. One of the most prevalent root diseases on wheat is take-all (Photo 1) and it is caused by the fungus
Gaeumannomyces graminis var. Tritici or Ggt. In Afrikaans the common term vrotpootjie has been adopted as a name for this disease.

The fungus also causes disease on triticale, barley and rye in the wheat production areas of the Eastern Free State and the Western Cape, as well as in the cooler irrigation areas in Vaalharts, Sandvet, Riet River and the Orange River.

In the Western Cape, the fungus survives on grasses and volunteer wheat. Crop residues are important for the survival of take-all, since it will be the source of infection for the following crop season. Take-all is the result of the Ggt-wheat-environment interaction and occurs only when favourable conditions of these three factors of plant, pathogen and environment overlap.

Infection starts when the roots of the wheat plant come in contact with the fungus. It spreads between and within the roots by means of dark runner hyphae. Once inside the roots, the phloem is destroyed, cutting off the supply of water and nutrients to the plants.

These are the symptoms usually used in assessing diseased plants. The plants will be shorter in length and lighter in colour, compared to healthy plants. Typical bleached ears will develop during the flowering stage in patches in the field and ears will die before the grains are fully developed.

As such, yield losses can be expected, depending on the amount of wheat infected. The best way to determine if it is take-all, is to pull out the plants to examine the stems and roots. Infected plants will be easy to pull out. Brown to black discolouration of roots, crowns and lower internodes of the plant will be visible. The roots are also brittle, thickened and break easily (Photo 2).

The nitrogen and magnesium content of the soil is of great importance in the development of take-all in plants, since low levels of the abovementioned can lead to an increase of infected plants. Phosphorus and potassium promote secondary root development and if these new roots grow vigorously, the plant will sustain less serious damage from take-all.

One of the main cultivation practices to control the fungus is crop rotation. The life cycle of the fungus is broken by cultivating a nonhost crop, and the amount of inoculum from the previous crop is reduced and fewer or no infection occurs in the following year. A one year fallow period could also be practiced to reduce the amount of inoculum in the field.

The disease intensity in the following wheat season will be determined by the amount of decomposition of the stubble from the previous year. Crop rotation with soybeans and lucern may indirectly increase the disease occurrence, since it reduces the manganese levels in the soil that promotes take-all. Oats is one of the crops that are recommended to reduce take-all.

Take-all will occur in areas that are over-irrigated and/or where soil is compacted (low drainage areas), which may lead to flooded areas in the field and where soil temperatures are between 5°C and 30°C. At lower temperatures, take-all infections may become more severe. The pH of soil is also very important since take-all is able to develop where the soil pH is between 5,5 and 8,5.

Drainage or structural problems that may restrict rooting should be corrected. High seeding rates which reduce root development by interplant competition and favour the disease should be avoided. An elevated soil pH is likely to give rise to deficiencies on minor nutrients, such as copper and manganese. Liming is important to prevent the development of acid patches in the field, but overliming should be avoided. In South Africa there are currently no resistant cultivars available to control take-all, due to the irregular occurrence of this disease.

Take-all is not the only reason why bleached heads occur. Other reasons may also include Fusarium head blight, cold damage, insect damage and drought and mistakes are easily made in identifying the problem.

Fusarium head blight needs a warm and humid climate whereas take-all prefers cooler and humid conditions. The differences between take-all and Fusarium head blight symptoms are displayed in Table 1.

For more information regarding take-all, contact Cathy de Villiers at the ARC-Small Grain Institute, Bethlehem on
058 307 3400 or DeVilliersC@arc.agric.za.