Importance of wheat seed treatment

Taking care of the health of the genetic material used in wheat crops is an essential task to obtain good results and avoid losses not only in the current harvest, but also in subsequent crops. Seed treatment is an alternative to at least minimize the incidence of diseases caused by fungi and bacteria.

To be successful in wheat farming, it is essential to take care of the health of the seeds, with special care for pathogens linked to these genetic materials, as they continue the biological cycle.

Seeds can harbor and transport microorganisms from all taxonomic groups, pathogenic or not. Therefore, the detection of these organisms becomes one of the most important tools in the phytosanitary management of diseases. The results of the tests used for this purpose may indicate the absence of pathogens or, if the pathogen is present, the level of inoculum in the seeds.

The incidence and severity of pathogens vary over time and depending on several factors, such as location, temperature and humidity. Therefore, continuous monitoring is important in order to qualitatively and quantitatively monitor the presence of these pathogens transmitted to wheat seeds (Brancão, 2008).

The adoption of joint measures, such as crop rotation, elimination of volunteer plants and secondary hosts, treatment and use of healthy seeds, use of resistant cultivars, application of fungicides and alternative products, make the control of wheat diseases more efficient ( Santos et al.

Of the phytopathogens found in wheat seeds, fungi are the most numerous and important that attack winter cereals. It is the case that I would alternate, extremely common and saprophytic fungus, known as alternaria spot, characterized by the occurrence of a black spot at the site of fungus penetration, with a yellowish halo around it that is not very prominent, but does not develop on the thicker veins and causes plant defoliation (Reis & Casa, 2012). 

Bipolaris sorokiniana

Bipolaris sorokiniana, cThe cause of common root rot, is responsible for brown spot, considered one of the most damaging diseases to wheat, with losses of 20% to 80% in yield (Barros et al, 2006). Under favorable conditions with temperatures of 20ºC and 28^oC and at least 15 hours of leaf wetness, the disease occurs throughout the crop cycle (Bacaltchuk et al.

As the disease progresses, the lesions become elliptical and generate abundant sporulation of the dark brown fungus, which gives the lesions a black appearance (Prates & Fernandes, 2001).

The primary inocula for this pathology are seeds generally with black tips, cultural remains of rye, barley, wheat, triticale, volunteer plants, secondary hosts and dormant conidia in the soil. They can be spread through rain, wind and contaminated seeds (Kimati et al.

Fusarium

Fusarium is caused by the fungus Gibberella zeae, that causes Fusarium head blight in wheat. The main inocula of these fungi are ascospores, which remain on crop residues and remain between one growing season and another due to the wide range of hosts and can be transported by wind over long distances and deposited on anthers causing infection (Brazil, 2009) . It can also be spread over short distances when conidia are carried by rain splash. 

Drechslera

Drechslera causes the yellow spot that is predominant in Rio Grande do Sul, favored by the direct planting system, which favors the survival and multiplication of the pathogen, as well as by the region's climate, with ideal temperature conditions between 18^oC and 28^oC and leaf wetness period greater than 30 hours. It is a necrotrophic fungus, therefore capable of surviving in cultural remains.

The most characteristic symptom is the yellow spot. The lesions are elliptical or diamond-shaped, usually with a yellow border and a dark brown center. Depending on the cultivar, greater or lesser chlorosis and necrosis may be observed, due to the action of toxins specific to the pathogen (Manning & Ciuffetti, 2005).

In addition to these fungi that affect plants in crops, it is necessary to observe storage fungi, such as Aspergillus, verticillium e Penicillium, which cause losses and make the cereal unfit for human or animal consumption.

Phytopathogenic bacteria can be associated with seeds both externally and internally. Contamination can occur in the crop, during the seed maturation period, when the inoculum produced on leaves or other parts of the plant reaches its surface, through rain splashes, irrigation water, wind, insects or during harvesting operations. , transport, processing and storage (Brazil, 2009).

Pectobacterium Symptoms caused by Pectobacterium are due to the production of large quantities of enzymes that degrade the plant cell wall. They vary according to environmental conditions, resistance or susceptibility of the cultivar and part 

Experiment with seed treatment

An experiment was carried out at the Phytosanitary Laboratory of Faculdade Ideau using Mirante wheat seeds treated and not treated with thiran + carbendazin in the ml/kg ratio in PDA (potato-dextrose-agar) and AA (water-agar) culture media.

The wheat seeds were separated into lots of 50 seeds from each method and placed in a gerbox. Afterwards, the media remained in a growth chamber at a temperature of 20^oC (± 2^oC), for a period of 12 hours of light and 12 hours of darkness for seven days, inducing seed germination and pathogen sporulation. After this period, the incidence of pathogen in each culture medium was analyzed.

In the first sample of seeds not treated in PDA medium, there was a high incidence of Pectobacterium. This can be explained by the lack of specific treatment for bacteria in seeds. The fungi with the highest incidence were Drechslerae bipolarisand storage Penicilliume aspergillus, because it lacks fungal treatment and because the seeds have been stored in bags since the previous harvest, often in inappropriate locations.

In sample 2 of seed not treated in AA medium, a lower incidence of Pectobacterium and also fungi. The explanation may lie in the fact that this medium is more used for the germination of spores, functioning as a humid chamber (Alfenas et al. 2007) and also because it has fewer nutrients for the development of organisms.

In sample 3 of seeds treated in PDA medium, a significant increase in Pectobacterium. According to Sexto (2013), bacteria can develop in environments where there is a lower incidence of fungi. A lower incidence of fungi was obtained due to the treatment of the seeds, but there was again the appearance of Bipolaris. According to Reis & Forcelini (1993), the black tip does not always appear, causing the seed to appear apparently healthy. However, the fungus is found inside the seeds, making sterilization difficult through treatment with fungicides that only affect the seed coat.

Sample 4 presented only fungi of the genus Bipolaris and again the incidence of bacteria, but in smaller quantities than in PDA medium.

From the results obtained in the experiment it was observed that a control efficacy of less than 100% is not sufficient and the infected seeds sown do not interrupt the parasite's life cycle.

Thus, the research shows the high incidence of pathogens in untreated seeds and their significant reduction in treated seeds, demonstrating the importance of using chemotherapy, which, although it does not eradicate the diseases, which would be very difficult, reduces the incidence. If these seeds were used in crops, there would certainly be a high level of economic damage threshold in the harvest and also in the cultural treatments for the next crops.

Ediane Roncaglio Baseggio, UFFS - Campus Erechim/Fapergs; Paloma Alves da Silva Sexto, Faculdade Ideau – Campus Getúlio Vargas, RS

Article published in issue 198 of Cultivar Grandes Culturas.