Sowing wheat in waterlogged soil increases the risk of mosaic

Frequent rains at the beginning of winter in the South Region can hinder the establishment of crops. Soil waterlogging, in addition to making it difficult for machines to enter, also increases the risk of incidence of the common wheat mosaic.

The cold and humid climate found in the Southern states favors the incidence of common wheat mosaic, a disease that depends on free water in the soil so that the virus vector can reach the root system of plants. Generally, the mosaic is seen in areas with a history of the disease – poorly drained land with water sources – as both the vector and the virus can remain in the soil for a long time. “The mosaic virus vector survives for a long time in soil. In research experiments, even leaving the soil at rest for five years, when we re-sow susceptible cultivars in that area, under favorable environmental conditions, the mosaic reoccurred, therefore total eradication is unlikely”, says Embrapa researcher Trigo Douglas Lau.

The waterlogging of the soil associated with the cold climate, which makes it difficult for water to evaporate quickly, forms the ideal environment for the mosaic. The forecast of rain, after sowing has been carried out in moist soil, also increases the risk of the disease occurring in areas with a history of mosaic.

Data collection from the last five years records that the greatest mosaic damage is associated with rainfall close to 200 mm in the month of sowing. “The most critical phase is in the days following sowing, when early infection has the potential to cause more damage to the plant, affecting the development of the forming tissues”, explains Douglas Lau. Damage can reach a 50% reduction in grain yield in susceptible cultivars, compromising ear formation in crops such as wheat and triticale. 

Symptoms

Mosaic symptoms begin to appear about a month after infection and are expressed until the end of the plant's cycle. The main characteristic of the disease is the alternation between healthy and affected tissues, forming green and yellow stripes on the leaves. Susceptible wheat plants may also show  severe reduction in growth and impaired heading.

The distribution of diseased plants in the field usually occurs in patches or patches, in places where soil drainage is not good. However, in years with a more favorable environment, the mosaic can infect the entire crop. The spread of the vector occurs over the long term, through machines that travel between contaminated areas and areas, until then, free of mosaic.

Control

Practices such as crop rotation and adequate nitrogen fertilization help reduce damage, but there are no products registered for chemical control of the vector.

“Measures such as crop rotation with plants such as oats reduce the incidence of mosaic, but they need to be used in conjunction with genetic resistance, especially in scenarios with high favorability to the disease”, explains Douglas Lau.

Therefore, the safest and most effective recommendation for producers is the use of mosaic-resistant cultivars. The research has already identified cultivars that produce well in areas with the virus. The results of experiments with the most planted cultivars or new releases for Rio Grande do Sul are published in the document “Reaction of wheat cultivars to common mosaic”, available on the Embrapa website.

Effectiveness of management practices for mosaic in wheat:

Management Practice	Efficiency
Crop Rotation	+/-
Genetic Resistance	+
Healthy Seeds	-
Sowing Time	+/-
Seed Treatment	si
Chemical Management	-
Biological control	si
Nitrogen Fertilization	+/-

Management PracticeEfficiencyCrop Rotation+/-Genetic Resistance+Healthy Seeds-Sowing Time+/-Seed TreatmentsiChemical Management-Biological ControliNitrogen Fertilization+/-

(-) ineffective; (+/-): not very effective; (+): effective; (si): no information. Source: Lau et al., 2020