Wheat Pests and Diseases

Crop protection measures against pests and especially diseases are usually necessary to protect the wheat crop’s health, yield, and grain quality. Wheat is susceptible to more than 30 diseases caused by fungi, viruses, and bacteria. The presence of a pathogen in a region, the severity of symptoms, and the level of the pathogen dispersal may vary significantly. For all wheat types (winter, spring, and durum), spring is the “hot” season for most disease problems, while there is also some risk for infection during the first half of autumn.

To minimize the effect of pests and diseases, the farmer needs to pay attention to the following:

Invest in preventing measures: This is of top importance, especially for bacteria and viruses. 

• Use resistant or tolerant varieties. Nowadays, plenty of varieties are resistant to one or more important wheat diseases and tolerant to many pests. Prefer to use varieties with resistance to the variants of the pathogen more prevalent in your area. 
• Use healthy (disease and pest free), clean, and certified seeds.
• The use of seed treatments (fungicides) may be an option in some cases – ask your local certified professional agronomist. 
• Adapt the sowing time.
• Implement crop rotation, ideally with species-crops having different “enemies” or are resistant to the most important wheat pest and diseases. This will reduce their population and starting pathogen load (inoculum).
• Keep the field weed-free and deal with crop residues. Weeds and crop residues can serve as disease and pest hosts and help in the overwintering of some important pathogens and pests. 
• Keep your plants vigorous. Avoid, if possible, any water stress and nutrient deficiencies. 
• Scout your field on regular basis and especially during periods with favorable environmental conditions for infection and dispersal (high humidity, mild temperature, etc.). 
• Know your “enemy’s” physiology, the favorable environmental conditions for its growth, and the ways and rhythm of dispersal.

In the case of known field history concerning a specific pest or pathogen, there might be a need for preventing or/and control measures. This may be mostly necessary in fungus diseases and soil-borne insects. Plant funguses and insects usually have high dispersal ability. As a result, a farmer may need to take action when suspicious symptoms or large insect populations are observed in neighboring (wheat) fields. Consult your local certified professional agronomist. 

• Act fast and with precision. Recognizing the diseases (pathogen) at an early stage can help you choose the correct product (control measure) and limit the dispersal and yield losses with minimum use of chemicals. If needed, send samples for analysis. 
• Have a planned in-crop fungicide regime. In general, fungicides are more effective as protectants. Remember to repeat the spaying when and for as long as needed.
• Know which pathogens or pests in your area have developed resistance to specific active compounds. 

The most common and important diseases in wheat

Most wheat growers invest in crop protection during spring (May) when the plants enter the reproductive stages. One of the most significant threats to yield is foliar diseases. The most common early-season foliar diseases are powdery mildew and Septoria leaf spot.

Powdery mildew

The fungus Blumeria graminis f. sp tritici  causes the disease and can reduce wheat yield by up to 25%. The growth of the pathogen is favored by cool (10-20 °C/ 59-72 °F) and wet weather (above 85% humidity) (1).

In the early stages, the symptoms appear as yellow flecks on leaves that later are covered with fluffy white powder. The symptoms usually start from lower leaves and progress upwards (sometimes in stems and heads). 

The disease is quite hard to be controlled once it is established in the crop. The management strategies include:

• The use of resistant varieties.
• Elimination of volunteer wheat and other plant hosts.
• Crop rotation.
• Prudent use of N-fertilizers.
• Use of seed dressings and foliar fungicides (2).

Usually, spraying with fungicides is applied at ear head emergence or appearance of disease on flag leaf (3). Ask your local certified professional agronomist. 

Septoria leaf blotch/spot

Septoria is a major problem for wheat crops in many areas. In severe infection from the pathogen Septoria tritici (Mycosphaerella graminicola), yield losses of 50% may occur. 

The most common symptom that appears early in winter is the elongated chlorotic lesion-spots on the leaves. In the lesion area, small black dots appear (pycnidia), and as the infection progresses, the leaf tissue becomes necrotic while spikes can also be affected. 

Resistant varieties are the primary protective measure against Septoria. Late sowing can also be helpful. Fungicide application is effective usually in the initial stages of the pathogen and can keep the disease in the lower leaves of the plants. Avoid using large amounts of chemical products since the pathogen can develop resistance quickly. Resistance of Septoria to strobilurin is nowadays widespread globally (4). 

Rust 

There are 3 main species of Puccinia that can affect wheat plants:

• Brown (leaves)?Puccinia recondite
• Black (stems)?Puccinia graminis f.sp. tritici
• Stripe-yellow (leaves, necks, and glumes)? Puccinia striiformis

The pathogens can survive mild winters, and the expression of symptoms is stronger in late fall or early spring or after flag leaf emergence (P. striiformis). Severe infections can lead to significant yield losses, reduced grain quality, and even crop failure. 

The disease is expressed by forming chlorotic-necrotic pustules on the infected plant part with yellow to orange or dark reddish-brown coloring dots (urediospores).

As managing measures, agronomists suggest using resistant wheat varieties to one or multiple Puccinia species. Additionally, the control of volunteer wheat during autumn, the use of treated seeds, timely sowing, and foliar spraying with fungicides are also effective (ask your local certified professional agronomist). The best protection is managed by combining all the above measures (5, 1).

Fusarium head blight (head scab)  

Caused by the fungus Fusarium graminearum (Gibberella zeae) is the most catastrophic wheat disease globally, with more than 50% yield reduction. 

The fungus infects the head of wheat plants during the flowering stage and produces dangerous mycotoxins (vomitoxin). The infected wheat heads become whitish, and the grains discolored, shrink, wrinkle, and lose weight. Grains intended for flour production (human consumption) should have less than 2 ppm vomitoxin (6). 

While earlier harvest can reduce the number of infected kernels, the toxin levels are lower when grains are harvested at 13–15% moisture (Simón et al., 2021). While several fungicides effectively control head scab, chemical control is not usually sufficient. The timely application of fungicides combined with the use of resistant cultivars, residue management, and crop rotation is expected to have the best results (7). 

Loose smut

It is a seed-borne disease and is more common in areas where seeds are horizontally distributed among the farmers. Yield losses are generally low, but they can reach up to 30% in severe infections. 

In the infected inflorescence-head, flowers and seeds are replaced by masses of smut spores that are blown away, leaving bare rachis-stalks. 

The best prevention measures are the use of resistant varieties and healthy clean seed with fungicidal dressing in combination with bioagent fungus (Trichoderma viride- 4 gm/kg seed). T.viride can also improve the initial vigor of the crop, and the treatment of the seeds should be done 72 hours before sowing, followed by the fungicide 24 hours before sowing. The measures mentioned above effectively control flag smut, too (3). You should always consult your local licensed agronomist. 

Crown rot

It is an important problem in many winter wheat fields, especially in northern regions, and more common in clay soils. The disease is usually caused by the fungus Fusarium pseudograminearum. 

The farmer should look for the characteristic brown color in the infected tillers’ base (up to 2-4 nodes). Whitehead formation can be observed. The plants lodge and are pulled up easily.

As management methods, agronomist often suggest the early sowing (winter wheat), crop rotation, residue management, adequate zinc nutrition and use of resistant wheat cultivars (8).

Bacterial leaf streak (black chaff)

The disease is caused by Xanthomonas campestris pv. translucens (Xanthomonas translucens pv. undulosa) and is more prevalent in warm, humid weather and in plants where the leaf tissue has been damaged. Yield losses are usually not severe but, in extreme cases, can reach 40%. The pathogen grows optimally at temperatures above 25 °C (78 °F) (9).

The characteristic of the disease is its sporadic appearance within a region of a field (symptoms in patches). Water-soaked lesions appear on the leaves, while in advanced infections, black chaff symptoms are detected on glumes.

All small cereals and grasses are pathogen hosts and should be avoided in a crop rotation system. Very few bactericides are available (copper-based). The most effective measures are the use of resistant varieties and healthy seeds and the management of crop residues and weeds.

Finally, there are no control measures for virus diseases. As a result, the protective measures are the farmer’s only option. The most effective measure is using resistant varieties and the control of volunteer wheat plants and insects that can carry and transmit the virus. While several viral diseases affect wheat, Barley Yellow Dwarf and Wheat Streak Mosaic are considered the most important. 

The most common and important pests in wheat

Insects do not usually consist a major problem for wheat crops. The severity of an infestation could vary from year to year, among different geographic regions, and can be affected by the crop species grown in the field. Monoculture of wheat or only small grain cereals could give rise to a larger, more dangerous population of wheat insects. Among them are Wheat midges, Hessian Fly, Wheat Stem Sawfly, Thrips, Aphids, Cutworms, Armyworms, Cereal Leaf Beetle, Helicoverpa species, and Mites. In a large population, root-knot Nematodes (Meloidogyne spp.) can also cause significant problems in all wheat types. 

Hessian Fly, Mayetiola destructor

It is considered one of the most destructive pests of wheat, and its population has been reported in most areas where the crop (winter, spring, and durum) is cultivated. Severe infestations have been reported in North Africa and the USA (Midwest states). Losses due to insect activity are more extended during the start of fall (10). The adults look like small mosquitoes. 

The damage in the plants is caused by the feeding of the larvae at the leaf sheath near the node. When the attack takes place in the seedling stage of wheat, it may result in plant death. If it happens after tillering, the plants weaken, the stems’ growth is affected, and they break close to harvest (11). 

To reduce the risk, you can choose to sow after the Hessian Fly free date (start of October for the North hemisphere, check them per country 12). In areas where Hessian Fly causes significant problems to wheat, no-tillage systems should be avoided, and volunteer wheat plants should be controlled. During spring, the farmer needs to scout his/her field for flaxseed (overwintering form of the insect = puparium), estimate the population, and decide if there is a need for immediate chemical control. There are some seed treatments and foliar insecticides registered for the insect. Still, due to the medium effectiveness of each measure, it is advised to be used in combination with other crop management practices. Crop rotation and prudent nitrogen fertilization can be beneficial. 

References:

1. https://www.canr.msu.edu/wheat/uploads/files/Wheat-101-report-2021-final%20-%20web.pdf
2. https://www.agric.wa.gov.au/spring/managing-powdery-mildew-wheat
3. https://iiwbr.icar.gov.in/wp-content/uploads/2018/02/EB-52-Wheat-Cultivation-in-India-Pocket-Guide.pdf
4. https://ahdb.org.uk/knowledge-library/septoria-tritici-in-winter-wheat
5. https://agriculture.vic.gov.au/biosecurity/plant-diseases/grain-pulses-and-cereal-diseases/leaf-rust-of-wheat
6. https://www.canr.msu.edu/wheat/disease/
7. https://www.udel.edu/academics/colleges/canr/cooperative-extension/fact-sheets/fusarium-head-blight-management-in-wheat/
8. https://grdc.com.au/resources-and-publications/grownotes/crop-agronomy/northernwheatgrownotes/GrowNote-Wheat-North-09-Diseases.pdf
9. https://wheat.pw.usda.gov/ggpages/wheatpests.html#common
10. https://ipm.ca.uky.edu/content/hessian-fly-wheat
11. https://extensionpublications.unl.edu/assets/pdf/g1923.pdf
12. https://extension.psu.edu/hessian-fly-on-wheat

Simón, M. R., Börner, A., & Struik, P. C. (2021). Fungal Wheat Diseases: Etiology, Breeding, and Integrated Management. Frontiers in Plant Science, 12, 498.

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Principles for selecting the best Wheat Variety

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Wheat Irrigation Requirements and Methods

Wheat Fertilizer Requirements

Wheat Pests and Diseases

Yield-Harvest-Storage of Wheat

Weed Management in Wheat Farming