Wheat powdery mildew, caused by the fungus Blumeria graminis f.sp. tritici, represents one of the main phytosanitary challenges for global and Brazilian wheat farming.
The pathogen, belonging to the ascomycete group, stands out for its high host specificity, biological complexity and adaptive capacity, establishing a sophisticated biotrophic relationship with the wheat plant that results in significant impacts on the productivity of this crop, which is essential for global food security.
Taxonomic position and specificity
Taxonomically, B. graminis belongs to the Kingdom Fungi, Phylum Ascomycota, Class Leotiomycetes, Order Erysiphales and Family Erysiphaceae.
The species has undergone taxonomic reclassification. It was previously called Erysiphe graminis. Its relocation to the genre Blumeria occurred in 1975, based on morphological and biological characteristics distinct from other members of the order Erysiphales.
A striking feature of this pathogen is its division into formae speciales, specialized lineages that, although morphologically similar, present high host specificity.
The special form tritici exclusively infects wheat (Triticum spp.), while others formae speciales , the I horded, avenae e dry are specialized in barley, oats and rye, respectively.
This specialization reflects a long coevolutionary process between pathogen and host, resulting in specific adaptations for colonization of particular hosts, which has important implications for disease management in different cereals.
Biological cycle and infection strategies
The life cycle of B. graminis f.sp. tritici comprises two complementary reproductive phases that ensure both efficient dissemination during the cultivation period and survival in adverse conditions.
The asexual phase (anamorph) predominates during the wheat growing season and is characterized by abundant production of conidia, while the sexual phase (teleomorph) occurs mainly at the end of the crop cycle or under unfavorable environmental conditions.
The infectious process begins when conidia are deposited on the leaf surface. Under favorable conditions of temperature (15-22°C) and high relative humidity, germination occurs rapidly, in approximately 1-2 hours. This process involves the sequential formation of specialized structures: first the primary germ tube, followed by the secondary germ tube that gives rise to the appressorium, a structure responsible for penetration into the host epidermis. This penetration occurs through a combination of mechanical pressure and enzymatic degradation of the cuticle and cell walls, involving cutinases, cellulases and pectinases secreted by the fungus.
After penetration, a sophisticated biotrophic relationship is established, with the formation of haustoria in the epidermal cells. These specialized structures invaginate the host cell plasma membrane without breaking it, creating an intimate interface for communication and nutrient translocation. The extrahaustorial matrix is formed between the haustorium wall and the host cell membrane, a critical region for nutrient absorption and secretion of effectors that modulate the plant's defensive responses.
Colonization proceeds with the development of superficial hyphae that form the characteristic grayish-white mycelium on the leaf surface. Within 3-5 days after the initial infection, erect conidiophores are formed perpendicular to the surface of the mycelium, which produce chains of conidia. These asexual spores are easily dispersed by the wind and can be transported for tens to hundreds of kilometers, initiating new infections. This asexual cycle can be repeated multiple times during the growing season, with intervals of 7-10 days between complete cycles, which explains the epidemic nature of the disease under favorable environmental conditions.
The sexual phase is completed with the development of cleistothecia, fruiting bodies that are initially whitish and gradually darken until they become dark brown or black. These structures contain asci with ascospores and function as important survival units during unfavorable periods. The following spring, under adequate humidity conditions, the cleistothecia rupture, releasing ascospores that can initiate new primary infections.
Pathogenic mechanisms and molecular interaction
The pathogenicity of B. graminis f.sp. tritici is based on sophisticated molecular mechanisms that allow the fungus to evade the host's defenses and establish a biotrophic relationship. As an obligate biotroph, the pathogen needs to keep the host cells alive, subverting the plant's defense mechanisms without triggering hypersensitive responses that would result in cell death.
In this context, the secretion of protein effectors that interfere in several plant defense signaling pathways, particularly those mediated by salicylic acid and jasmonic acid, stands out. These effectors modulate the host's cellular metabolism, redirecting nutrients to the haustorium and suppressing immune responses. The haustorium functions as a metabolic sink, preferentially absorbing glucose, fructose and amino acids through specialized transporters expressed in its membrane.
Molecular interactions between pathogens and hosts often follow a gene-for-gene model, in which pathogen avirulence (Avr) genes correspond to host resistance (R) genes. Notable examples include the avirulence gene AvrPm3a2/f2, recognized by wheat resistance alleles Pm3a and Pm3f, and AvrPM2, recognized by Pm2. Recent studies indicate that Brazilian isolates of B. graminis f. sp. tritici present a high frequency of virulence (≥95%) for Pm2 and Pm3a, suggesting that these resistance genes are no longer effective in the region. On the other hand, more recent genes such as Pm25, Pm35, Pm37 and MlAG12 present a low frequency of virulence (≤6%), indicating that they are still effective against Brazilian isolates.
The great genetic variability of B. graminis f.sp. tritici, resulting from both sexual reproduction and mechanisms such as somatic recombination and high mutation rate, allows the fungus to adapt quickly, overcoming genetic resistance introduced into commercial cultivars.
Dispersion and epidemiology
The spread of B. graminis f.sp. tritici It occurs predominantly via the air, through conidia produced abundantly during the asexual phase. These spores are easily detached from the conidiophores by air currents, with a peak release generally between 10 am and 14 pm, when relative humidity decreases after humid periods. This release mechanism, associated with the lightness of the conidia, allows dispersal over long distances.
Several environmental factors modulate the spread and establishment of the disease. Temperatures between 15°C and 22°C associated with relative humidity above 70% provide ideal conditions for conidial germination. Interestingly, unlike many other fungal pathogens, B. graminis f.sp. tritici does not require free water for germination, being favored by conditions of high humidity without intense precipitation, which could remove the conidia from the leaf surface.
The epidemiology of powdery mildew is characterized by the development of initial foci that rapidly expand under favorable conditions. The relatively short latency period (7-10 days) and the massive production of conidia give the disease a high epidemic potential, particularly in dense crops that hinder air circulation and favor humid microclimates.
Impact on Brazilian wheat farming
In the Brazilian agricultural context, wheat powdery mildew has gained prominence in the last six seasons, with more frequent and severe epidemics, especially in the South and Central-West regions. This increase is attributed to milder and wetter winters, ideal conditions for the development of the pathogen, as well as to the widespread use of susceptible cultivars. Recent studies indicate that the frequency of virulence for older resistance genes, such as Pm2, Pm3a, Pm4a, Pm4b, Pm8 and Pm17, is extremely high (≥95%) among Brazilian isolates, reflecting the loss of effectiveness of these genes. In contrast, more recent genes, such as Pm25, Pm35, Pm37 and MlAG12, still present low frequency of virulence (≤6%), suggesting that they are promising for use in genetic improvement programs.
The economic impact of the disease is manifested through the reduction of productive potential, with losses that can reach between 20% and 79% in conditions favorable to the disease, especially when the infection occurs in the early stages of plant development. In specific studies, losses of 32% were observed in the BR 23 cultivar and 79% in the OR 1 cultivar, both susceptible to powdery mildew. In addition to direct losses in productivity, powdery mildew can affect grain quality, resulting in depreciation of the final product.
Pathogen management
The management of this disease in Brazilian wheat farming is fundamentally based on three complementary strategies: genetic resistance, chemical control and cultural practices. With regard to genetic resistance, it is essential to prioritize the incorporation of more recent resistance genes that are still effective against Brazilian isolates, such as Pm25, Pm35, Pm37 and MlAG12, in new cultivars, to ensure the durability of resistance. Genetic improvement programs led by institutions such as Embrapa have developed cultivars with different levels of resistance to the pathogen, representing a sustainable and economically viable strategy for producers.
Chemical control, through the application of fungicides from the triazole and strobilurin groups, is an important alternative, particularly in situations of high disease pressure or when using susceptible cultivars. However, the continued use of these molecules has selected populations of the pathogen with reduced sensitivity. Recent studies have demonstrated the loss of sensitivity of Brazilian isolates of B. graminis f.sp. tritici to the fungicide triadimenol, used as seed treatment, which explains control failures reported by producers since the 2008 harvest in southern Brazil. This highlights the importance of monitoring the pathogen's sensitivity to fungicides and adopting integrated management strategies, including rotation of different chemical groups and the use of mixtures, to delay the development of resistance.
Cultural practices, including crop rotation, adequate management of planting density and nitrogen fertilization (which can favor the development of the disease when excessive), complement the arsenal of measures available to Brazilian wheat growers.
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