**1. Introduction**

182 Soybean Physiology and Biochemistry

Werner, D. (1987). Pflanzliche und Mikrobielle Symbiosen. Georg Thieme Verlag Stuttgart-

Soybean *Glycine max* L. Merr. is cultivated in several tropical and subtropical regions of the world. United States (USA) and Brazil are the world's largest producers and exporters of oilseed (Agrianual, 2008; Miyasaka & Medina, 1981).

Despite the high production and export of Brazilian soybeans, many factors have affected the quality or quantity of production of that crop, causing reduction in financial returns per unit area, such as disease epidemics. Among the diseases, powdery mildew, whose etiologic agent is *Microsphaera diffusa* Cke. & Pk., suddenly began to cause significant damage in soybean, despite having a broad host range and have been reported in Brazil, Canada, Republic of China, India, Puerto Rico, South Africa, United States (Sinclair, 1999), Germany, Argentina and Bolivia (Sartorato & Yorinori, 2001) .

According to Yorinori & Hiromoto (1998), crops widely affected by the disease, had estimated reductions between 30 and 40% of yield, in the same order of magnitude as those reported abroad by Dunleavy (1978) and Philips (1984). The susceptibility of cultivars and the influence of climate favored epidemics with high rates of disease progress, in successive years in Brazil. Considering the lack of resistance of most of the cultivars, chemical control is required, especially in the south and the high plains of the savannah biome (Sartorato & Yorinori, 2001). In 1996/97, epidemics of powdery mildew in soybean in a great extent of Brazil, from the Central West region to the Rio Grande do Sul state, resulted in average losses of 15 and 20% in susceptible cultivars, with extremes ranging from 50 to 60% (Yorinori & Hiromoto, 1998 ; Seganfredo & Silva, 1999).

*M. diffusa* is distinguished from *M. polygoni* by presenting cleistothecium with appendages forked at its end (Sartorato & Yorinori, 2001; Grau, 1975). The fungus is an obligate parasite that develops throughout the soybean shoot, including leaves, stems, petioles and pods. Symptoms can range from chlorosis, green islands, rusty spots, defoliation or severe combination of these symptoms, depending on the reaction of cultivars. Chlorotic spots and necrosis on the leaf veins indicate a hypersensitivity reaction. However, the most obvious is the very structure and powdery white fungus on the surface of infected parts (Yorinori, 1982; Yorinori, 1986, Tanaka et al., 1993; Yorinori et al., 1993; Sinclair, 1999; Sartorato &

<sup>\*</sup> Edson A. Pozza2, João de C. do B. Costa3, Josimar B. Ferreira4, Dejânia V. de Araújo5, Luiz Gonsaga de Carvalho2, Fábio Moreira da Silva2 and Luciana Sanches1

*<sup>1</sup>Federal University of Mato Grosso, Brazil, 2Federal University of Lavras, 3CEPEC/CEPLAC l, 4Federal University of Acre, 5State University of Mato Grosso, Brazil* 

Yorinori , 2001). In general, the lower leaves of young plants are more susceptible than the upper leaves (Mignucci & Lim, 1980).

In relation to physiological changes in the host, Mignucci & Boyer (1979) studied the inhibition of photosynthesis and transpiration of soybean infected with powdery mildew and found lower photosynthesis and transpiration with increased infection. With 82% of leaf area infected, more than half of the leaf photosynthetic activity had been lost and transpiration dropped to 36% compared to control, considering the direct result of the change in metabolic activity induced by the pathogen. Because infection occurs primarily in the lower leaves and poorly lit, it is unlikely that the reduction in rates of photosynthesis and leaf transpiration resulted in great reduction in soybean yield, however, favorable climatic conditions may enabled the infection of upper leaves leading to high losses (Mignucci & Boyer, 1979; Sartorato & Yorinori, 2001).

Susceptibility of cultivars and influence of the climate has caused outbreaks of powdery mildew in successive years in Brazil. The lack of resistance in most cultivars have required chemical control mainly in the south and the high plateaus of the savannahs. In the U.S.A., powdery mildew caused economic damage reached in the 70's and early 80's. Since then, the use of resistant cultivars has dispensed chemical control (Sartorato & Yorinori, 2001).

Reactions of different soybean varieties to powdery mildew and the effect of environmental variables in the progress of the disease have been reported (Arny et al. 1975; Buzzell et al. 1975; Degree & Laurence, 1975, Johnson & Phillips, 1961; Mignucci 1977; Mignucci & Boyer, 1979; Mignucci & Lim, 1980; Lohnes & Bernard, 1992; Lohnes & Nickell, 1994). According to Bedendo (1995), in Brazil, powdery mildew may occur in the humid and cold climates, but are favored by hot dry conditions (20-25 °C). According to the author, conidia do not germinate when is present a film of water on the leaf surface, however, relative humidity near 95% is required for germination.

Mignucci et al. (1977) reported temperatures of 18 ºC as favorable to the development of powdery mildew on susceptible cultivars and at temperatures of 30 ºC disease progress was inhibited. Degree & Laurence (1975) also observed lower disease severity at 30 °C. According to Sartorato & Yorinori (2001) the information about the effects of relative humidity, leaf wetness, rainfall, solar radiation or other environmental factors in the progress of powdery mildew in soybeans was not precise.

Therefore, the intensity of powdery mildew of soybean under different temperatures and periods of leaf wetness on the cultivars conquista and suprema was evaluated.
