Wheat is affected not only by fungal infections, but also by bacterial and viral diseases caused by highly specialised pathogens, which often pose a significant challenge to crop protection, as many of these viruses can infect a wide range of wild grass species. The broad host range among wild flora creates persistent reservoirs of infection, making disease management in cultivated crops considerably more difficult.

4.12.1 Bacterial Diseases

The pathogens responsible for the disease are bacteria with a rod-like shape, which belong to the genera Pseudomonas, Xanthomonas, and Bacillus.

4.12.1.1 Black or basal bacterial blight

The causative agent of this disease is Pseudomonas syringae pv. atrofaciens (synonym P. atrofaciens). The bacteria are motile rods measuring 0.5-0.8 × 1-2.5 micrometers with rounded ends and one polar flagellum. When grown on potato agar, the bacteria produce colonies that are bright yellow and have a slimy texture. The disease impacts all above-ground portions of the wheat plant. On the leaves, small, dark green, water-soaked spots first appear, which later darken to brown or black. On the stems, dark brown or black stripes develop along the length of the plant. Lesions in the spikelets appear at the base of the glumes, starting off as translucent spots. In more advanced stages, the entire ear may become misshapen and darken, while the stem may bend, and the grains often dry out and shrink. Seedlings that sprout from infected seeds usually fail to grow properly and often perish. This disease is especially common in spring wheat cultivated in areas with higher levels of humidity.

4.12.1.2 Bacterial leaf spot

The disease is caused by Xanthomonas campestris pv. translucens. It primarily affects the leaves, initially appearing as water-soaked lesions that eventually turn brown.

The disease becomes most apparent during the flag leaf stage and continues through the heading and flowering phases of wheat growth. During this time, the symptoms appear as pale, watery streaks along the central vein of the leaves. As the disease progresses, these streaks darken, and a yellowish substance from the bacteria forms on the surface. In dry weather, this exudate dries up and crumbles away. It is thought that insects, especially those that feed on plant sap like aphids, thrips, and cicadas, are key contributors to spreading the infection.

4.12.1.3 Mucilaginous or yellow bacterial blight 

The causative agent of the disease is the bacterium Rathayibacter tritici (Carls. et Vidav) Zgurskaia et al. (syn.: Clavibacter iranicus (Carls. et Vidav), Corynebacterium tritici Hutch., Phytomonas tritici Elliott, Agrobacterium tritici Savul.).

The disease impacts both the leaves and their sheaths, where long, pale or yellowish stripes first appear and eventually cause the leaves to curl. In plants that have been infected, the ears become significantly distorted, often staying within the leaf axils and eventually transforming into a formless, gelatinous mass. In humid weather, a yellowish exudate forms on the affected plant parts, which dries and turns white in dry weather. Infected plants often do not produce ears and may die. The pathogen is a rod-shaped bacterium with rounded ends, able to move, and has a single flagellum at one end. It doesn’t produce spores and typically measures between 0.8 and 2.4 by 3.2 micrometers. On meat-peptone agar, it forms yellow, round colonies that are slightly raised, while on potato agar, the colonies appear pale yellow. This bacterium is able to reduce sugars like glucose, fructose, and glycerol, but doesn’t metabolise mannitol or salicin, and cannot break down gelatin or starch. It doesn’t generate hydrogen sulfide, but it does release ammonia. One key feature that sets it apart from other plant pathogens is its Gram-positive reaction. The ideal temperature range for its growth is between 20-30 °C, and it is unable to survive at temperatures above 50 °C (Israelsky, 1979).

The infection can spread through seeds that are infected, and it's also carried by a nematode called Anguina tritici. However, even though this nematode is present in North America, cases of mucilaginous bacterial blight haven't been seen in wheat there. The disease can also be transported by the wind or insects, which helps it move from one plant to another.

Without a host plant to feed on, the bacterium can survive for quite some time on leftover plant material and can travel long distances through irrigation water.

With Kazakhstan joining the WTO and increasing trade with countries in Asia and Africa, the chances of introducing new pathogens have grown. Therefore, wheat seeds coming from regions where mucilaginous bacterial blight is common should be carefully inspected and planted in a controlled quarantine area for monitoring. To avoid spreading the disease, the seeds can be treated by either heating them to temperatures of 50-55°C or using a disinfecting agent to eliminate any bacteria. If the disease does spread, the affected wheat should be removed or treated with a non-selective herbicide, and the soil should be turned over to a depth of at least 20-25 cm to reduce the risk of further contamination.

4.12.2 Viral Diseases

Wheat can be affected by a range of diseases, such as the Russian mosaic on winter wheat, striped mosaic on spring wheat, and red stripe on sorghum leaves. The pathogens behind these conditions are highly specialised and can affect various grasses, both cultivated and wild, along with their main host. Some of these viruses are passed on through seeds, while others spread when infected and healthy plants come into contact with each other or when tools and insects carry the virus. Viruses are typically divided into three types based on how they spread: persistent, semi-persistent, and non-persistent. Non-persistent viruses are easily spread through mechanical means. Aphids are known to spread many viral diseases, often after just a short contact (1-2 minutes) with an infected plant. On the other hand, persistent viruses are transmitted by leafhoppers, which become infected after feeding on a plant for several hours and can spread the virus after an incubation period. Semi-persistent viruses are somewhere between these two in terms of transmission. Additionally, some conditions that cause yellowing, which may look similar to viral infections, are actually caused by mycoplasma bodies. These are different from viruses because they can grow on nutrient-rich artificial media and are typically round or oval, measuring about 200-300 nanometers in size (Vlasov, Larina, 1982).

4.12.2.1 Dwarfism disease

A characteristic sign of the disease is the excessive tillering of infected plants, significant stunting in growth, and dwarfism. The vector of the disease is the leafhopper Laodelphax striatellus Fall., which overwinters on the stubble of wild grasses and infects spring grain crops in the spring, as well as various species of millet and other grass species.