Okra [Abelmoschus esculentus (L.) Moench], is an economically important vegetable crop grown in tropical and sub-tropical parts of the world. This crop is suitable for cultivation as a garden crop as well as on large commercial farms. The area under okra cultivation in India is 526.00 (000 ha) with production of 6460.00 (000 MT) & productivity of 12.10 MT/ha (Anonymous, 2022). In Gujarat, okra is grown throughout the year, providing a continuous and good source of income to the farmers. It is mainly grown in Vadodara, Surat, Junagadh, Banaskantha and Bhavnagar districts. 

It occupies an area of 75.27 thousand hectares with a production of 921.72 thousand metric tonnes, having an average productivity of 10.90 MT/ha (Anonymous, 2022).

Many factors responsible for yield loss of the crop, one of them is the diseases are major constraints for low yield of okra (Sastry and Singh 1974). A number of fungal, bacterial, viral diseases have been reported in India. Among the fungal diseases affecting okra crop, powdery mildew caused by Erysiphe cichoracearum DC is most important disease causes considerable yield losses. The occurrence of the disease has been reported from Mexico (Diaz-Franco, 1999). In India, the disease has been reported to occur in Delhi (Prabhu et al., 1971), Karnataka (Sohi and Sokhi 1973), Himachal Pradesh (Raj et al., 1992) and Maharashtra (Jambhale and Nerkar 1983). The disease initiates as white minute patches first on the upper surface of lower older leaves and then spreads to younger ones. Grayish white powdery coating is visible on severely affected leaves. Leaves finally show necrosis resulting in withering, drying and defoliation. It is not appearing on stem, branches and fruits. Powdery mildew affects plants of all growth stages and may result yield losses to the tune of 17 to 86.6 per cent (Sridhar and Sinha 1989). Crop yield losses are significant under favorable weather conditions if the infection takes place in early stages of plant growth (Gupta and Thind 2006). The loss due to powdery mildew is proportionate to the disease intensity and varies considerably depending on the stage of the plant growth at which disease occurs. In southern regions of the state and other parts of the country this powdery mildew pathogen is known to produce only the conidial or anamorphic stage. The identification of the pathogen is therefore done only on the basis of the anamorphic stage.

Weather factors, mainly temperature and relative humidity above a threshold limit, are key factors for the occurrence of powdery mildews. Development of powdery mildews is characterized by an initial slow growth when favorable conditions just set in during susceptible stage of the host. But suddenly an epidemic starts with a devastating appearance, leaving no time for taking any appropriate control measures (Rajput et al., 2025). The fungi causing powdery mildews are very fast-growing (4 to 5 days latent period) pathogens, therefore, timely decision is vital for their management (Sharma et al., 2022). Except for grape and mango, there is no appropriate forewarning system for powdery mildews on other crops. In the absence of any forewarning information, growers suffer heavy yield loss (González-Domínguez et al., 2023). 

Epidemiology encompasses the study of the factors associated with disease development. The three major components of the disease viz., susceptible host, virulent pathogen and favorable weather in time and space causes epidemic (Singh, 1996). Accurate forecast of the crop age at first appearance of the disease and the risk of a mildew epidemic would enable farmers to decide on the optimum timing of fungicide sprays and to avoid unnecessary pesticide application. Hence, the present study was undertaken to find out the relationship of meteorological parameters with disease development.

The field experiment was conducted at College Farm, N.M.C.A., N.A.U., Navsari during all three seasons (Kharif 2014, Rabi 2014-15 and Summer 2015). The crop was kept totally unprotected where no any application of fungicides /botanicals was given. All the agronomical practices were followed as per the recommendation.

To determine the influence of various physical factors of environment on the development of the powdery mildew of okra, the data on powdery mildew intensity were correlated with different meteorological parameters viz., maximum temperature (Max. Temp.), minimum temperature (Min. Temp.), average temperature (Av. Temp.), morning relative humidity (RH1), afternoon relative humidity (RH2), average relative humidity, rain fall (RF), sunshine hours (SH) and wind speed (WS) through analysis of correlation regression. The corresponding data on various weather parameters were collected from department of meteorology, N.M.C.A., N.A.U., Navsari. The powdery mildew intensity was recorded at weekly interval adjusting standard week in the field of susceptible variety GAO-5. The per cent disease intensity for powdery mildew were recorded by using formula proposed by Wheeler (1969). Powdery mildew intensity observed during the crop season at standard week’s interval was correlated with the D-7 (Days prior to observation) weather parameters. For the regression equations, the multiple step wise regression analysis was carried out.

In kharif season initiation of the disease was first noticed after 59 days of sowing (1.56%). After that, the linear progress of the disease was observed up to the maturity of the crop. The disease was appeared (1.56 %) at flowering stage (35th SMW) and found highest (71.11 %) at maturity (42nd SMW). The disease development was recorded higher during 38th to 41st SMW. The disease progress was found higher during 66 to 87 days of crop age i.e. at fruiting period. During Rabi season, the initiation of the disease was noticed after 45 days of sowing. Moreover, the disease progress was found higher from 6th to 11th SMW. The disease was increased up to 76.00 from 6th to 12th SMW. In summer season, the disease was initiated after 66 days of sowing in both the varieties. The maximum disease intensity, 30.89 per cent was noticed. The disease was started from 22nd SMW and higher disease increasing trend was found from 23rd SMW to 24th SMW. 

Comparatively, powdery mildew intensity was found higher during rabi and kharif seasons than the summer. The disease appearance was also found earlier (45 DAS) in rabi season, whereas, late appearance in summer season (66 DAS). The rise of per cent disease intensity in the initial stage of disease was very slow. This was followed by rapid progress of the disease due to favorable weather conditions and reaches maximum (Fig. 3 & 4) at maturity stage. The similar confirmation was given by Solanki et al. (1999) who found that powdery mildew intensity proportional with the crop age in powdery mildew of mustard.

Looking to the season wise, the disease was increased faster during 38th to 41st SMW in kharif, 6th to 11th SMW in rabi and 23rd to 24th SMW in summer. Moreover, there was negligible rainfall occurred in kharif & rabi during fruiting stage. In summer season, due to high rainfall during susceptible stage (fruiting stage) of crop very slow development of the disease was recorded. 

In high rainfall areas and periods, other diseases become more prevalent but powdery mildews decreases as rainfall increases (Agrios, 2005). Light rainfall may induce high spore dispersal. Two main mechanisms are involved, the splash effect (Gregory, 1961) and the rain tap and puff effect (Hirst & Stedman 1963). 

Correlation coefficient values (Table 1) reflect the extent of association between PDI with different meteorological factors, crop age and heat unit. Out of  twelve factors, maximum temperature (0.903), mean temperature (0.728), bright sun shine hours (0.910), GDD (0.746), AGDD (0.962) and crop age (0.959) were found significantly positive correlation while minimum temperature (-0.840), morning  relative humidity (-0.845), afternoon relative humidity (-0.935) and mean relative humidity (-0.929) were showed significantly negative correlation with the powdery mildew intensity of okra. During rabi season correlation coefficient values showed significantly positive relationship of PDI with minimum temperature (0.730), GDD (0.795), AGDD (0.986) and crop age (0.970). Whereas, in summer among all the variables, afternoon relative humidity (0.845), mean relative humidity (0.824), AGDD (0.949) and crop age (0.942) were registered significantly positive correlation while maximum temperature (-0.821), mean temperature (-0.755)  BSSH (-0.907) and GDD (-0.756) showed significantly negative correlation. The results obtained from present study are in confirmation with the reports of earlier workers. Solanki et al. (1999) stated that mean temperature, bright sun shine hours, GDD, AGDD and crop age were significantly and positively correlated with powdery mildew disease intensity and progress on mustard crop. These results are also in conformity with the observations of Gupta et al. (2001); Kohire et al. (2008).

The result is moderately similar with the confirmation of Singh et al. (2008) who noted that maximum temperature were significantly and positively correlated, whereas bright sun shine hours and wind speed negatively correlated with powdery mildew disease intensity and AUDPC. While, Vikas and Ratnoo (2013) observed that sunshine hours positively and rainfall negatively correlated with powdery mildew development. Maximum and minimum temperature, morning and afternoon relative humidity and rainfall significantly and positively correlated with powdery mildew disease intensity reported by Singh and Pannu (2015).

To formulate the simple and effective linear regression equation for powdery mildew disease prediction, the various meteorological factors, heat unit (GDD and AGDD) and crop age were assessed. The maximum, minimum and mean temperature replaced by GDD and AGDD when heat unit were used in regression analysis. An independent variables viz., average of 7 previous days (D-7) of meteorological factors from initiation to maturity of crop heat unit (GDD and AGDD) and crop age with dependent variable PDI at respective SMW were computed by multiple step wise regression analysis.

Generally, the pathogen becomes most active over the susceptible host under the congenial environmental conditions. These important factors were critically studied during all the three seasons viz., kharif, rabi and summer of 2014-15. 

The data of kharif season (Table 2) showed that meteorological factors viz., afternoon relative humidity (X5), mean relative humidity (X6) and crop age (X10) were identified as a most responsible factors for the disease development under natural conditions. Variation accounted by this regression equation is 97.5 per cent (R2 = 0.975). The obtained regression equation for the prediction of PDI (Y) is as under:

Y= -168.2943 – 3.4904 (X5) +4.552 (X6) -0.8580 (X10)                                                                           (1)

Whereas, in rabi season (Table 3) meteorological factors viz., morning relative humidity (X4) and crop age (X10) were most crucial factors for the disease development under natural conditions (Fig. 3). Variation accounted by this regression equation is 96.8 per cent (R2 = 0.968).The obtained regression equation for the prediction of PDI (Y) is as under:

Y= -4.9313 -0.7027 (X4) + 1.3092 (X10)                   (2)

In summer season (Table 4)  meteorological factors viz., mean temperature (X3) bright sun shine hours (X8) and crop age (X10)  were found as most associative factors for the disease development under natural conditions (Fig. 4). Variation accounted by this regression equation is 99.7 per cent (R2 = 0.997). The obtained regression equation for the prediction of PDI (Y) is as under:

Y= 30.4522 – 1.7341 (X3) – 1.3610 (X8) + 0.6045 (X10)                                                                          (3)

In case of okra powdery mildew, the rain tap effect might be important both quantitatively and qualitatively for two reasons. First, germination of conidia is negatively affected by the presence of free water (Willocquet  & Clerjeau 1998). Thus, the germination of conidia dispersed by rain splash may be hampered compared with that of dry spores. Second, powdery mildew colonies are mostly located on the lower surface of leaves, especially at the beginning of the growing season. As these colonies are not likely to be hit by rain drops, the rain tap effect may be the most important mechanism of spore dispersal by rain. In the okra, thus light rainfall can be considered as favorable to the fungus, as it would allow dispersal without the drawbacks associated with heavy or continuous rain (Sharma et al., 2022). 

Under continuous rain, the conidial stock is likely to decrease and conidia may be washed off from leaves, leading to a decrease in spore concentration, as observed on 24 to 27 SMW in summer season. Such decreases because of rain have been reported for other powdery mildews (Willocquet and Clerjeau 1998; Hammett & Manners 1971; Sutton & Jones 1979; Pauvert, 1986). Continuous rainfall is also likely to be unfavorable to okra powdery mildew because of physical damage to leaf surface mycelium and conidiophores. 

In present study the optimum maximum and minimum temperature for maximization of powdery mildew severity on okra leaves were 36.7 and 22.5°C for kharif and 12.9 to 34.4°C for rabi under field conditions was in confirmation to Bhattacharya and Shukla (2002) who have pointed out that optimum temperature for development of powdery mildew severity were 37.4 and 21.4°C under irrigated conditions. 

E. cichoracearum from cantaloupe showed a temperature optimum of 25° to 28°C on the basis of in vivo length of hyphae reported by Yarwood et al. (1954). They also observed that growing mycelia are apparently more sensitive to high temperature than are quiescent conidia. The LD 50 of 37°C air temperature for a 3-day infection of E. cichoracearum on cucumber was about 10 hours, while with freshly inoculated leaves it was about 20 hours. 

The short life of powdery-mildew conidia is associated with their high water content (Yarwood, 1950), but E. cichoracearum is more tolerant of high temperature than other Erysiphe spp. because it has lowest water containing conidia among the powdery mildews and has the longest-lived conidia at ordinary temperatures (Yarwood et al., 1954).

Wide variation (10-15°C) in the maximum and minimum temperature and day and night relative humidity (30-40%) increases powdery mildew intensity in cucumber (Gupta et al., 2001). They also reported that too high and too low temperature did not favor powdery mildew development. The optimum temperature and relative humidity for conidial germination is 25°C and 100 per cent, respectively. 

Production of conidia is optimal at 20°C and declines rapidly above and below that temperature (Ward and Manners 1974). Although conidia only survive for several days, they are capable of disseminating the fungus long distances. New pustules with conidia are produced every seven to ten days at optimal conditions and provide repeating cycles of spores. Conidia germinate most rapidly at 97 to 100 per cent relative humidity, but their high water content allows them to germinate when humidity declines below 50 percent. However, germ tube growth and appressorium production are greatly reduced below 92 per cent relative humidity (Friedrich et al., 1993). 

According to Jyothi et al. (2014) conidia of powdery mildew germinated at 65 per cent and 100 per cent humidity, indicated the ability of fungus to infect both under dry and humid conditions. Similarly, Jacob et al. (2008) reported that conidia of Oidium lycopersici germinate well at 65 to 100% RH and best at 95%. They also reported that low light intensity was associated with optimal germination. The bright sunny periods militate against the development of the fungus, and have a lethal effect on detached spores (Peries, 1962).

Bhattacharya and Shukla (2002) concluded that increase in temperature and sunshine duration had positive effect on powdery mildew severity, whereas, relative humidity was negative under both irrigated and rainfed conditions. High association of powdery mildew severity with sunshine duration could be possible by its indirect effects as has been suggested by Calhoun (1973) or due to changes in sun's inclination resulting increase in temperature with sunshine duration and evapo-transpiration (Rotem, 1978). Germination decreased or was delayed in the presence of free water demonstrated by Butt (1978).

While, according to Kohire et al. (2008) dry and cool environmental conditions that means temperature range between 12.2 to 22.8°C and low relative humidity from 30.2 to 48.8 % ideal for conidial germination.

The result was also more or less similar with the findings of Vikas and Ratnoo (2013); Singh et al. (2008).  

From the results of the present investigation, it is very clear that powdery mildew of okra is high to moderately occurring in Navsari district during throughout the year. The infection on leaf produced huge quantity of inoculum in the presence of favorable climatic conditions. Due to presence of heavy load of inoculum, the pathogen attacked on all the leaves (Lower, Middle and Upper, respectively). So, the weather during the period between 38st to 42nd, 6th to 11th and 23rd to 24th SMW were observed extremely crucial for development of the powdery mildew.

Critical value (0.05) = +/- 0.666 (Kharif), +/- 0.602 (Rabi) and +/- 0.754 (Summer)

Table 2: Regression analysis of per cent disease intensity of powdery mildew with different meteorological factors and crop age in kharif season (Average of D-7).

Standard error of est.= 5.3923

*‘b’ significant at 5% level and std. error of variable is given in parentheses

Table 3: Regression analysis of PDI of powdery mildew with different meteorological factors and crop age in rabi season (Average of D-7).

Standard error of est.= 5.8375

Table 4: Regression analysis of PDI of powdery mildew with different meteorological factors and crop age in summer season (Average of D-7).

  Standard error of est.= 0.9985

Fig. 1. Disease occurrence during different crop age in all three seasons.

Fig. 2. Relation of disease intensity  with afternoon relative humidity, mean relative humidity and crop age (Kharif).

Fig. 3. Relation of disease intensity with Morning relative humidity and crop age (Rabi).

Fig. 4. Relation of disease intensity with bright sun shine hours and mean temperature and crop age (Summer).