Efficacy of novel fungicide combinations against downy mildew in cucumber and their influence on biochemical defense mechanisms

Introduction

Cucumber (Cucumis sativus L.) is important vegetable crop which belongs to gourd family Cucurbitaceae and consisting of 118 genera and 825 species (Lebeda and Cohen, 2011) [6] . It is the second most popular cucurbit after watermelon and the most extensively planted crop after tomatoes, cabbage and onions (Tatlioglu, 1993) [15] . It grows well in the world's tropical and subtropical regions as well as plains and hills (Shetty et al., 2002) [12] . India is the primary origin of cucumbers. Cucumber farming dates back 2000 years in China and 3000 years in India, according to archaeological data. According to De Candolle (1882) [5] , China is regarded as a secondary centre of cucumber genetic diversification. Vitamins "B" and "C" and minerals including calcium, phosphorus, iron and potassium are abundant in cucumber. Silica, a trace mineral that fortifies connective tissues, is abundant in it. Because it includes 95% water, it helps rehydrate the entire body and meets daily vitamin requirements. Cucumber contains 96.3 g of fluids, 0.4 g of proteins, 0.1 g of fat, 0.3 g of minerals, 0.4 g of dietary fibre, 2.4 g of carbohydrates, 10 mg of calcium, 25 mg of phosphorus, 1.5 mg of iron, 0.03 mg of thiamine, 0.2 mg of niacin, 7 mg of vitamin "C" and 13 K cal. (Yadav and Rai, 2005) [9] . Cucumbers are now grown in greenhouses, small gardens and commercial farms of all sizes all over the world (Hossain et al., 2010). Over 75% of the world's cucumber production comes from China. India produces 1651.92 thousand Mt of cucumbers on 116.62 thousand hectares of land (Anon., 2020-21). West Bengal (322.29 Mt), Madhya Pradesh (252.26 Mt) and Karnataka (122.98 Mt) are the three states that produce the most cucumbers. These three states account for approximately 41.21 percent of the nation's land area and 42.22 percent of its production (Anon., 2020-21). A number of diseases brought on by bacteria, viruses and fungi afflict cucumbers. One of the most economically significant and pervasive plant infections in many parts of the world is Pseudoperonospora cubensis (Berkeley and Curtis), which causes downy mildew of cucumbers (Palti and Cohen, 1980) [8] . The disease is found everywhere cucurbits are grown, including in semi-arid regions of the Middle East as well as temperate and tropical regions. The infection has a significant impact on both quality and yield in these producing zones and it is particularly harmful in places with warm, humid weather that is conducive to disease growth (Thomas, 1996) [16] . It is estimated that downy and powdery mildew infections cause a 50-70% yield loss (Sitterly, 1972) [13] . Crop loss was reported to be between 50 and 70 percent despite the application of fungicides, crop rotation and fall ploughing, among other cultural measures (Awad, 2000) [4] . During the 1998 crop season, the incidence and magnitude of downy mildew varied from 50 to 100% and 20 to 78%, respectively (Sharma et al., 2003) [11] . According to Vyavahare (2020) [17] , the incidence of downy mildew in various Maharashtra districts was between 11.87 and 48.47 percent in 2018 and 19.56 and 60.13 percent in 2019. Downy mildew is most problematic in moist and humid climate of North Gujarat; generally, cucurbits are grown in large scale throughout the year. Therefore, due to intensified cropping of cucurbits, downy mildew causes havoc losses.

Farmers of North Gujarat prefer either age old metalaxyl and copper-based fungicides or some systemic fungicides. This experiment of the disease is of practical significance, since the pathogen disseminates through air, disease spreads very fast. Hence, identification of the natural factors to assess the infection to the crop plant and its management through the novel fungicide available in Indian market.

Materials and Methods

In order to manage cucumber downy mildew in the field, the current study was conducted in the summer of 2023 at the Vegetable Research Station, SDAU, and Sardarkrushinagar. Eight treatments, including the control, have been incorporated in the Randomised Block Design experimental design employed for the study. For every experiment treatment, a 5 x 6 m 2 plot was maintained. There was total eight treatments including the control. The application of fungicides was done at 15 days interval. Where, first spray was done on the first appearance of disease. The total seven fungicides tested together for their efficacy those are: Ametoctradin 27% SC + Dimethomorph 20.27% SC @ 0.2%, Metalaxy l8% + Mancozeb 64% WP @ 0.2%, Cymoxil 8% + Mancozeb 64% WP @ 0.2%, Chlorothalonil 33.1% + Metalaxyl M 3.3% SC @ 0.7%, Dimethomorph 50 WP @ 0.1%, Metiram 55% + Pyraclostrobin 50% WG @ 0.2% and Azoxystrobin 23 SC @ 0.25%. Water was the only substance utilised in the control plot. Details of the treatment are provided in Table 1. Fruits were picked and block-by-block observations were made to assess each treatment's yield and B:C ratio independently. Both treated and untreated plots' yields were noted and calculated in tonnes/ha. Five randomly chosen plants per plot were used to measure the prevalence of downy mildew in order to assess the effectiveness of treatments. The observations were obtained on the 7 th , 14 th , 21 st , 28 th and 35 th days following the initial spray, as well as before the 24-hour mark.

Percent disease intensity

The following formula provided by Wheeler (1969) [18] was used to calculate the% disease intensity of downy mildew.

Sum of numerial rating PDI =

x 100 Total no. of samples observed x Maximum of rating scale

Percentage disease control

The percentage disease control (PDC) was calculated according to formula given by (Mathur et al., 1971) [7] .

PDI in Control -PDI in treatment PDC = PDI in control

Area under disease progress curve (AUDPC)

Using the method provided by Shaner and Finnay (1977) [10] , the plant disease per cent disease index was determined at seven-day intervals to determine AUDPC.

AUDPC =

Where, Xi and Xi + 1 are severities on date i and date i + 1, respectively ti is the number of days in between date i and date i + 1n is the number of observations recorded.

Biochemical analysis

To determine how the fungicidal treatment affected the activity of these biochemicals, biochemical analyses of peroxidase, polyphenol oxidase and chlorophyll content were conducted before and after spraying at intervals of one, five, and ten days. Peroxidase, polyphenol oxidase, and chlorophyll activity were measured using methods described by Yadav et al. (2018) [19] , Taneja and Sachar (1974) [14] and Arnon (1949) [3] . The relevance of the final evaluations was then determined using statistical analysis. The computer program "OPSTAT," created by O.P. Sheoran, CCS HAU, Hisar, was also used to investigate the data collected throughout the research. .43 percent over control and 133.00 q/ha, 130.14 q/ha, 125.22 q/ha and 121.54 q/ha, respectively. 80.27q/ha was the lowest yield observed in the control plot.

Result and discussion

Following treatment spraying, peroxidase activity was measured in each treatment and ranged from 0.13 U gm-1 FW to 0.18 U gm-1 FW. The leaves sprayed with metalaxyl 8% + mancozeb 64% WP (0.18 Ugm-1 FW) had the highest peroxidase activity, whereas the plot treated with fungicide dimethomorph 50% WP (0.11 U gm-1 FW) had the lowest.

In contrast to the control, the other treatments had demonstrated notable variations. In all treatments, the range of activity increased one day and five days after spraying, while the peroxidase activity showed a little decrease ten days after spraying, as indicated in table 3 and figure 2.

Polyphenol oxidase activity

Following treatment spraying, the polyphenol oxidase activity in each treatment was measured and found to vary between 0.10 and 0.18 U gm-1 FW. The leaves treated with metalaxyl 8% + mancozeb 64% WP had the highest peroxidase activity (0.18 U gm-1 FW), whereas the leaves treated with azoxystrobin 23% SC had the lowest (0.10 U gm-1 FW). In contrast to the control, the other treatments had demonstrated notable variations. In all treatments, the range of activity increased one day prior to and five days following spraying, while the polyphenol oxidase activity slightly decreased ten days following spraying, as indicated in table 4 figure 3.

Chlorophyll content

When chemical treatments were used in the field following disease incidence, the amount of chlorophyll increased slightly compared to what was seen before to the chemical spray. The plot sprayed with metalaxyl 8% + mancozeb 64% WP (1.83 mg/g fresh leaf) had the highest chlorophyll content, whereas plants treated with azoxystrobin 23 SC (1.69 mg/g fresh leaf) had the lowest. Significant variations in the chlorophyll content were also observed in the other treatments when compared to the control, as indicated in table 5 and figure 4.

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