**3. Results and discussion**

Symptoms of powdery mildew evaluated at 9, 11, 13 and 15 days after inoculation, were characterized by chlorosis, green islands, rusty stains, and combination of these symptoms in the cultivars suprema and conquista. However, the most striking characteristic evaluated was the presence of the fungus, and powdery white structure on the surface of infected leaves. This symptomatology was consistent with those described by Sartorato & Yorinori (2001), Tanaka et al. (1993), Yorinori et al. (1993) and Yorinori & Hiromoto (1998). According to Sartorato & Yorinori (2001) and Yorinori et al. (1998), may also be variations in the symptoms of powdery mildew due to climatic variations, genetic variability between populations of *M. diffusa*, genetic resistance of cultivars, stage of plant development and adopted agronomic practices. Tanaka et al. (1993) studying the occurrence of powdery mildew (*M. diffusa*) in a collection of 27 soybean genotypes, in a green house, observed differences of severity symptoms presented by By Hampton cultivars (more susceptible), followed by IAC-Foscarin 31 and IAC-Santa Maria 702, respectively. According to Lohnes & Bernard (1992), Lohnes & Nickell (1994) and Mignucci & Lim (1980), the differing responses of powdery mildew in soybeans are consequences of three alleles at locus Rmd: Rmd-c (resistant), Rmd (resistance in adult plants) and rmd (susceptibility) and according to Dunleavy (1978) and Phillips (1984), these differences may be evidenced by a 35% loss of productivity in soybean cultivars susceptible to powdery mildew in the field.

A significant interaction in the F test between temperature and leaf wetness was observed for the AUDPCS in conquista cultivar (P = 0.0242) and the isolated effect of temperature in conquista (P <0.0001) and suprema (P <0.0001) . Thus, models were adjusted using nonlinear regression to describe the monocyclic process of the epidemic based on the dependent variables. With regard to temperature, a greater amount of disease was observed at temperatures around 23 °C for the conquista and 24 °C for the suprema cultivar. Temperatures above 15 ºC and 30 ºC were not favorable to the development of powdery mildew in both cultivars (Figures 2, 3 and 4).

Likewise Leath & Carroll (1982) in a study on powdery mildew of soybean cultivars, evaluating 38 cultivars, observed greater susceptibility of cultivars Ware, Falcon, AP350, V76-438, Emerald, AgDSR232, AgDSR532, Md71-583 as well as smaller and larger disease progress (*M. diffusa*) in Georgetown, under temperature of 29.6 °C and 23.2 °C, respectively. However, Mignucci et al. (1977) found at temperatures of 18 ºC in a green house, the greater progress of powdery mildew in Flambeau, Norchief, Chippewa 64, Corsoy, Harosoy 63, Wells cultivars,, grown in the USA and Puerto Rico. Seedlings were subjected to temperatures of 18, 24 and 30 °C per 14 hours, with alternating 10 hour temperature of 20 °C, to simulate day and night temperatures. In further studies, the same authors, after inoculation of *M. diffusa*, in cultivar Harosoy, in a green house, diseased plants were kept in a growth chamber under daytime temperature of 26 ± 2 ºC and night 21 ± 2 ºC (Mignucci & Chamberlain, 1978) and at 25 ± 0.25 °C ( Mignucci & Boyer, 1979). However, both Mignucci et al. (1977), Mignucci (1989) and Leath & Carroll (1982) agreed that temperatures around 30 ºC were not favorable to disease progress, similar to that observed in the cultivars

The significant variables in the F test by the variance analysis of AUDPCS were subjected to regression analysis, linear and nonlinear adjustment models (Leite & Amorim, 2002, Reis et al., 2004). In the case of significant interaction, the combined effect of temperature and leaf

Symptoms of powdery mildew evaluated at 9, 11, 13 and 15 days after inoculation, were characterized by chlorosis, green islands, rusty stains, and combination of these symptoms in the cultivars suprema and conquista. However, the most striking characteristic evaluated was the presence of the fungus, and powdery white structure on the surface of infected leaves. This symptomatology was consistent with those described by Sartorato & Yorinori (2001), Tanaka et al. (1993), Yorinori et al. (1993) and Yorinori & Hiromoto (1998). According to Sartorato & Yorinori (2001) and Yorinori et al. (1998), may also be variations in the symptoms of powdery mildew due to climatic variations, genetic variability between populations of *M. diffusa*, genetic resistance of cultivars, stage of plant development and adopted agronomic practices. Tanaka et al. (1993) studying the occurrence of powdery mildew (*M. diffusa*) in a collection of 27 soybean genotypes, in a green house, observed differences of severity symptoms presented by By Hampton cultivars (more susceptible), followed by IAC-Foscarin 31 and IAC-Santa Maria 702, respectively. According to Lohnes & Bernard (1992), Lohnes & Nickell (1994) and Mignucci & Lim (1980), the differing responses of powdery mildew in soybeans are consequences of three alleles at locus Rmd: Rmd-c (resistant), Rmd (resistance in adult plants) and rmd (susceptibility) and according to Dunleavy (1978) and Phillips (1984), these differences may be evidenced by a 35% loss of

wetness duration in disease intensity was modeled (Reis et al., 2004).

productivity in soybean cultivars susceptible to powdery mildew in the field.

mildew in both cultivars (Figures 2, 3 and 4).

A significant interaction in the F test between temperature and leaf wetness was observed for the AUDPCS in conquista cultivar (P = 0.0242) and the isolated effect of temperature in conquista (P <0.0001) and suprema (P <0.0001) . Thus, models were adjusted using nonlinear regression to describe the monocyclic process of the epidemic based on the dependent variables. With regard to temperature, a greater amount of disease was observed at temperatures around 23 °C for the conquista and 24 °C for the suprema cultivar. Temperatures above 15 ºC and 30 ºC were not favorable to the development of powdery

Likewise Leath & Carroll (1982) in a study on powdery mildew of soybean cultivars, evaluating 38 cultivars, observed greater susceptibility of cultivars Ware, Falcon, AP350, V76-438, Emerald, AgDSR232, AgDSR532, Md71-583 as well as smaller and larger disease progress (*M. diffusa*) in Georgetown, under temperature of 29.6 °C and 23.2 °C, respectively. However, Mignucci et al. (1977) found at temperatures of 18 ºC in a green house, the greater progress of powdery mildew in Flambeau, Norchief, Chippewa 64, Corsoy, Harosoy 63, Wells cultivars,, grown in the USA and Puerto Rico. Seedlings were subjected to temperatures of 18, 24 and 30 °C per 14 hours, with alternating 10 hour temperature of 20 °C, to simulate day and night temperatures. In further studies, the same authors, after inoculation of *M. diffusa*, in cultivar Harosoy, in a green house, diseased plants were kept in a growth chamber under daytime temperature of 26 ± 2 ºC and night 21 ± 2 ºC (Mignucci & Chamberlain, 1978) and at 25 ± 0.25 °C ( Mignucci & Boyer, 1979). However, both Mignucci et al. (1977), Mignucci (1989) and Leath & Carroll (1982) agreed that temperatures around 30 ºC were not favorable to disease progress, similar to that observed in the cultivars

**3. Results and discussion** 

evaluated in this study. In another pathosystem, powdery mildew of grape (*Uncinula necator* (Schw.) Burr.), The optimum temperature for growth of the fungus was 25 °C, while in the temperature between 21 and 30 °C there was germination of spores and increased sporulation. At temperatures above 33 ºC occurred death of spores and colonies (Thomas et al., 1994; Reis, 2004).

Fig. 2. Nonlinear regression of the progress curve of disease severity (AUDPCS) of powdery mildew of soybean in conquista cultivar according to the interaction between temperature and leaf wetness.

With regard to leaf wetness in conquista cultivar, there were signs of the disease from 0 to 8 hours of leaf wetness, with growth up values of AUDPCS until 8 hours, with the maximum of temperature of 23 ºC, indicating the need of water for germination of spores and fungus infection. From that point, higher values of leaf wetness reduced the AUDPCS (Figure 2). With respect to the isolated effects of leaf wetness periods in suprema cultivar, the maximum point of leaf wetness in the AUDPCI occurred in the period of 12.9 hours, with significant reduction near 0 and 24 hours (Figure 5). There is little information in the literature about the effects of leaf wetness on powdery mildew in soybeans (Sartorato & Yorinori, 2001), however, according to Bedendo (1995), this disease can occur in humid regions, but is favored by dry environments. Mignucci (1989) reported that the low relative humidity is highly favorable precisely described the development of powdery mildew in soybeans, though not presented values to describe precisely. Similarly, Brodie & Neufeld (1942) studying the development of conidial structures of *Erysiphe polygoni* DC., found germination in relative humidity ranging from 0-100%, while Mattiazzi (2003) studying the effect of mildew on the soybean production, observed greater progress at a relative humidity of 80%. Thus, the relative humidity of the growth chamber, with an average of 50%, may have given the conidial germination, even in treatments with no leaf wetness.

Fig. 3. Nonlinear regression of the progress curve of disease severity (AUDPCS) of powdery mildew of soybean in conquista cultivar according to the isolated effect of temperature.

Fig. 4. Nonlinear regression of the progress curve of disease severity (AUDPCS) of powdery mildew of soybean in suprema cultivar according to the isolated effect of temperature.

Fig. 5. Linear regression with polynomial quadratic fit of the progress curve of disease incidence (AUDPCI) of powdery mildew of soybean in suprema cultivar (Y axis) according to the isolated effect of leaf wetness (X axis).

Therefore, the results on the effect of leaf wetness in the progress of powdery mildew (*M. diffusa*) are contradictory and studies on the interaction effect between temperature and duration of leaf wetness on disease progression had not yet been assessed. The results presented potential use in prospective studies on the effects of weather on the progress of powdery mildew of soybean cultivars in Brazil.

### **4. Conclusion**

188 Soybean Physiology and Biochemistry

humidity of 80%. Thus, the relative humidity of the growth chamber, with an average of 50%, may have given the conidial germination, even in treatments with no leaf wetness.

Fig. 3. Nonlinear regression of the progress curve of disease severity (AUDPCS) of powdery mildew of soybean in conquista cultivar according to the isolated effect of temperature.

Fig. 4. Nonlinear regression of the progress curve of disease severity (AUDPCS) of powdery mildew of soybean in suprema cultivar according to the isolated effect of temperature.

The progress of the severity of powdery mildew (AUDPCS) in suprema and conquista cultivars was favored by air temperatures around 23 °C and 24 °C, respectively.

Leaf wetness of 8h and air temperature of 23 ºC provide the maximum progress of disease severity in conquista cultivar.

Temperatures above 30 °C and 15 °C reduced the intensity of the disease.

#### **5. Summary**

In this study the effects of temperature and leaf wetness period on the intensity progress of powdery mildew in soybean conquista and suprema cultivars were evaluated. Plants at the V3 stage were inoculated in greenhouse. Subsequently, the plants were conditioned in growth chambers at temperatures of 15, 20, 25 and 30°C and leaf wetness periods of 0, 6, 12, 18 and 24 hours. Severity data was integrated in time by the disease progress curve for incidence (AUDPCI) and severity (AUDPCS). Non-linear regression models were adjusted for the disease severity and a polynomial fit was adjusted for disease incidence data. Temperatures near 23 ºC and 24 ºC favored the powdery mildew intensity progress (AUDPCS) in Conquista and Suprema cultivars, respectively. Leaf wetness period of 8 h allowed the maximum progress of the disease in conquista at temperatures of 23 ºC. Temperatures near 30 ºC and 15 ºC reduced powdery mildew intensity. The maximum point of leaf wetness in the AUDPCI occurred in the period of 12.9 hours, with significant reduction near 0 and 24 hours

### **6. References**


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