Management of yellow stunt disease in wheat

Among the viruses that affect wheat crops is yellow dwarfism, caused by Barley yellow dwarf virus (BYDV) has the potential to cause serious damage, such as delay in plant development, decreased ability to withstand environmental stress and consequent decrease in productivity due to the reduction in the number and weight of grains. When available, genetic resistance is an important ally in the management of this disease.

Virosis is a disease caused by viruses. Just as occurs in humans and animals, plants can be infected by virus species that are capable of multiplying in their cells, producing disorders that result in visible symptoms and reduced production. Wheat is host to many species of viruses. In Brazil, two viruses are most common and have been causing damage to wheat farming since its expansion in the 1970s to the present day: yellow dwarfism, caused by virus species Barley yellow dwarf virus (BYDV) and Cereal yellow dwarf virus (CYDV), and the common wheat mosaic, attributed to Soil-borne wheat mosaic virus (SBWMV). Recently introduced in Brazil, the Wheat streak mosaic virus (WSMV) which is transmitted by the wheat curl mite (Aceria tosichella). As plant viruses can hardly be treated in crop conditions, virus control measures are preventive, with the aim of preventing or hindering the arrival of viruses in the crop and its spread. Genetic resistance is an efficient form of virus control and should be used when available.

The names of plant viruses usually refer to the host and the typical symptoms resulting from the infection. In case of Barley yellow dwarf virus (BYDV - Luteovirus), the original description occurred in barley and the symptoms evident in this host are a strong yellowing of the leaves (which occurs from the apex to the base of the leaf), reduced growth and delayed plant development. Growth reduction is not restricted to height (dwarfism), the number of tillers, leaf mass and root system mass can also be reduced. Among the most characteristic symptoms of this virus is the change in the color of the leaf blade. In wheat, yellowing of the leaf blade generally occurs, but, depending on the cultivar, other more reddish tones can be observed. In addition to the change in color, morphological changes occur, with the leaf blade taking on a lanceolate appearance and becoming more rigid. The set of these morpho-physiological changes can lead to a delay in plant development (increased time needed to complete the cycle) and make the plant less capable of withstanding environmental stresses, such as water deficit. The decrease in productivity is due to the reduction in the number and weight of grains. The expression of symptoms is variable and depends on the level of susceptibility and/or tolerance of the cultivar and the time in which the plants were infected. The earlier the infection occurs, the more severe the symptoms tend to be. Therefore, the symptoms are not always evident and can only be perceived comparatively between infected and non-infected plants. They are generally observed in groups of plants (reboleiras), which correspond to the areas of multiplication and dispersion of the aphid vector. 

BYDV species (Luteovirus) and CYDV (polerovirus) are transmitted by aphids (Hemiptera, Aphididae). Of the various species of BYDV (BYDV-PAV, BYDV-PAS, BYDV-MAV) and CYDV (CYDV-RPV, CYDV-RPS), BYDV-PAV predominates in Brazil. In autumn, this virus can easily be found in black oats (whose leaves turn reddish and the leaf blade is curled). It is also common to find the oat (or stem) aphid on oat plants. Rhopalosiphum padi, one of the most efficient BYDV-PAV transmitters. This virus is also transmitted by aphids that occur in more advanced stages of wheat, such as Metopolophium dirhodum (leaf aphid) and Sitobion avenae (ear aphid). When feeding on the sap of an infected plant, the aphid acquires viral particles, which migrate through its digestive tract and hemocoel, accumulating in the salivary gland. The virus is retained in the aphid's phase changes, but is not transmitted to its progeny. Transmission occurs when the viruliferous aphid feeds on a healthy plant. The virus is not transmitted by other insects, seeds, soil or mechanically. During autumn and spring, when temperatures are mild and aphid populations are numerous, aphids migrate from infected grasses to healthy grasses, allowing transmission of the virus. For Brazilian conditions, aphids may migrate from oat crops to newly planted wheat crops. It is also possible for aphid migrations to occur between wheat crops. As a function of climate conditions, wheat is planted at different times, as in some regions the crop enters maturity, aphids can migrate to others where wheat is at younger stages of development.

The potential damage of this aphid-virus complex to wheat production results from the interaction of the components: incidence of the disease resulting from epidemiological conditions and the level of tolerance/resistance of the cultivars. Dry years with high temperatures (temperatures of 18ºC-25°C) favor aphid populations, allowing an increase in the incidence of the virus. To control aphid populations, cultural practices (avoid the presence of reservoir plants), chemical and biological control can be combined. Biological control, carried out mainly by species of microimenoptera (“wasps”), has been very successful in Brazil, considerably reducing the aphid population. Chemical control can be carried out in seed treatment and aerial part applications. The recommended action levels are 10% of plants with aphids in the vegetative phases and 10 aphids per tiller/ear in the reproductive phases.

Even though wheat cultivars are susceptible (which are infected by the virus), they show variation in their reaction to BYDV. Thus, one can opt for those that are more tolerant to infection, especially in regions with a warmer climate and, therefore, more prone to yellow dwarfism epidemics. With the aim of providing information on the reaction of wheat cultivars to BYDV-PAV, Embrapa Trigo carries out an annual evaluation of cultivars that make up the State Trial of Wheat Cultivars of Rio Grande do Sul (EECT-RS). The tests are carried out under standardized conditions.

In each trial, 34 wheat genotypes are evaluated, the 30 EECT-RS cultivars of the respective year and four controls (BRS Timbaúva and BR 35, tolerant to BYDV; Embrapa 16 and BR 14, intolerant to BYDV). As a vector it is used R. padi, carrying a BYDV-PAV isolate. The viral inoculum is multiplied in black oat plants and used to create R. padi viruliferous. The test is carried out on a screen at Embrapa Trigo, Passo Fundo, Rio Grande do Sul, between July and November. Wheat cultivars are sown in the first week of July in plastic pots (capacity of 7 liters). After emergence, thinning is carried out, keeping five plants per pot. For each cultivar, five pots are subjected to inoculation (infestation with R. padi viruliferous). Another five pots are not inoculated and serve as a witness to the development pattern and productive potential of the genotype under the conditions under which the test is conducted. Inoculation is carried out at the two-expanded leaf stage. The pots to be inoculated are transferred to another screen, where each of the plants receives a leaf fragment, with 10 aphids, positioned at the intersection between the two leaves. Two days later, a new infestation is carried out on plants containing less than 10 aphids. The period for virus transmission is one week, after which insecticide is applied. After the death of the aphids, the inoculated pots are transferred to the initial screen and, for each genotype, five pairs are formed, composed of an inoculated pot and a non-inoculated pot, which are randomly distributed in the screen area. Top dressing nitrogen is applied in the form of urea (80kg/ha) at the tillering stage. During the test, insecticides and fungicides are applied to prevent the occurrence of insects and diseases. Visual assessments of symptoms are performed at the stalk elongation and sprouting stages. The visual assessment of symptoms is carried out by comparing the height and mass of the aerial part, estimating the reduction that the set of inoculated plants presents in relation to the set of non-inoculated plants for each of the five pairs of pots of each cultivar. Grades are assigned according to the following scale: 1 = 0% to 20% reduction; 2 = 21% to 40% reduction; 3 = 41% to 60% reduction; 4 = 61% to 80% reduction and 5 = reduction greater than 81%. At the end of the test, each pot is harvested separately and the total weight of grains for each experimental unit (pot) is determined. Comparisons are made using the weight of grains produced per pot (g/pot). The damage caused by BYDV-PAV on grain productivity is estimated for each cultivar by comparing the “Inoculated Plants” (I) treatment with the “Non-Inoculated Plants” (NI) treatment. Damage% = (NI - I) / (NI) *100, where: NI = weight of grains/pot for treatment non-inoculated plants; I = weight of grains/pot for treatment inoculated plants.

For the cultivars that made up the 2014 trial, all those evaluated showed symptoms, with average scores varying between 1,9 and 5 (Table 1). For most cultivars, there was variation in the score assigned between replications. In general, plants with a score equal to or greater than 4, in addition to the pronounced reduction in height and mass of the aerial part, also showed a delay in the development cycle and severe yellowing of the leaves. This group includes Mirante, the only cultivar with a score of 5 in all replications, and the cultivars BR 14 (only one score below 5), Estrela Átria, CD 1550, Jadeíte 11, FPS Nitron, Ametista and TBIO Sintonia. The cultivars that demonstrated the least effect of the virus with average scores below 3 were LG Oro, BR 35, BRS Parrudo, Tec Frontale and ORS Vintecinco. The damage to grain productivity caused by BYDV-PAV, on average, was 44,4%. The frequency distribution was: damage 0-20% = 2,9% (1); 21-40% = 35,3% (12); 41-60% = 58,8% (20); 61%-80% = 0% (0) and 81%-100% = 2,9% (1). The largest reduction observed was 88,9% (Mirante) and the smallest was 20,1% (ORS Vintecinco) (Table 1). The correlation between the productivity of inoculated vessels and that of non-inoculated vessels was 0,64. The correlation between visual assessment of symptoms and damage % was 0,73. The correlation between NI and damage % was -0,21, and the correlation between I and damage % was -0,87. The only cultivar with damage between 0%-20% was ORS Vintecinco (Table 1). The productivity obtained from the non-inoculated pots of this cultivar was close to the average, and the productivity from the inoculated pots above the average +1 standard deviation. The cultivars LG Oro, BRS Parrudo, TEC Frontale, and BR 35 also stood out positively as a group that presented damage between 21%-40%. The cultivars with productivity above the average (+1 standard deviation) in the presence of the virus were ORS Vintecinco, Tec Frontale, LG Oro, Fundacep Horizonte and TBIO Pioneiro. The cultivars with productivity below the average (-1 standard deviation) in the presence of the virus were Mirante, CD 1550, BR 14, TBIO Sintonia. Considering the combination between visual note of symptoms and productivity of inoculated plants, the cultivars Mirante, CD 1550, BR 14 and TBIO Sintonia were those that proved to be most intolerant to BYDV-PAV infection. Using the same criteria, the cultivars LG Oro, TEC Frontale and ORS Vintecinco proved to be the most tolerant to infection.

According to trials carried out since 2011, the percentage damage to wheat cultivars due to viral infection ranged between 44,4% and 49,9%. In table 2, estimates of % Damage caused by BYDV-PAV can be compared for all cultivars that were present in the state trial during this period.

Douglas Lau, Paulo Roberto Valle da Silva Pereira, Ricardo Lima de Castro, Embrapa Trigo

Article published in issue 206 of Cultivar Grandes Culturas.