Management of Powdery Mildew in Cluster Bean using Fungicides, Botanicals and Bioagents

Cluster bean [Cyamopsis tetragonoloba (L.) Taub.] is an important legume crop belonging to family Fabaceae. It is a drought tolerant crop suitable for cultivation under rainfed conditions in arid and semi-arid regions of India. The sweet and tender young pods are consumed as a vegetable or snacks in north-western and southern India and the mature seeds can be eaten during food shortages (Pachundkar et al., 2013). The green and tender pods are cooked as favorite vegetables in many parts of the country including South India. The green pods serve as a nutritious vegetable which contains water (82.5%), protein (3.7%), carbohydrate (9.9%), fat (0.2%), fibre (2.3%), other minerals (1.4%) viz. calcium (0.13%), phosphorus (0.25%), iron (5.8 mg/100 g) and vitamins (49 mg/100 g) (Deore et al., 2004). Young pods, fresh or dry forage are used as livestock feeds. The plant is also used as a green manure and cover crop. It yields up to 45.00 t ha^-1 of green fodder, 6.00-9.00 t ha^-1 of green pods and 0.70-3.00 t ha^-1 of seeds (Anonymous, 2010). India is the largest producer of guar seeds with 80 per cent of total production in the world and Rajasthan is leading state producing 75 per cent of total production of India. In India, area under cluster bean cultivation is 5.15 million hectares and production of 2.46 million tonnes with a productivity of 478 kg ha^-1 (Anonymous, 2017). In Karnataka it is cultivated round the year in limited area in districts like Dharwad, Belagavi, Vijayapura, Haveri etc. for tender vegetable pods.
       
The cluster bean crop is affected by various biotic and abiotic stresses which are responsible for its poor quality and low yield resulting in severe economic losses to the country as it is an important cash crop with a great potential for foreign exchange. Among biotic stresses, the damage caused by fungal foliar diseases is one of the major constraints. Among the foliar diseases, powdery mildew caused by Leveillula taurica is an important disease causing the yield loss upto 50-55 per cent (Channamma et al., 2015a).The disease manifests mainly on leaves and pods. Severely affected plants are defoliated and weakened by premature drying and death of infected leaves. The incidence of powdery mildew is more observed in the regions where crop season is prolonged. Warm temperature (33°C or above), high humidity (more than 80 per cent) and bright sunshine are congenial conditions for disease development (Channamma et al., 2015b).
       
Though there is large area under cluster bean cultivation in India, the productivity levels are low because of incidence of diseases in general and powdery mildew in particular. The studies carried on cluster bean are very few and as such there is no information related to powdery mildew disease. By considering the seriousness of disease and the economic losses caused by the disease, the present investigation was carried out to formulate suitable management practices against powdery mildew.

The experiment was conducted at Main Agricultural Research Station, University of Agricultural Sciences, Dharwad during Rabi 2017. The recommended package of practices viz., Farm yard manure at the rate of 20 t ha^-1 and fertilizers Urea, SSP (Single Super Phosphate), MOP (Muriate of Potash) at the rate of 25, 75 and 60 kg ha^-1, respectively were used. The whole amount of SSP and MOP; half amount of urea were applied as basal dose before sowing of seeds. The rest amount of urea was applied at 30 days after sowing (DAS). The experiment was laid out in a randomized complete block design (RCBD). The seeds of cluster bean cv. Pusa Navbahar were sown on the main field by following spacing of 30×15 cm and with plot size of 3 m× 3 m. Light irrigation was given immediately after sowing.
       
Based on the studies of Vijaykumar et al., (2021), Wettable sulphur (Sulfex 80% WP), hexaconazole (Contaf 5 EC), tebunconazole 50% + trifloxystrobin 25% WG (Nativo75% WG), nimbecidine (Azadirachtin 0.03%) and Bacillus subtilis (MT383652.1) were selected for field experiment.
       
Two sprays were given as per the combination and schedule along with unsprayed control to know their efficacy in managing the powdery mildew under natural epiphytotic condition. The first spray was done immediately after the onset of disease (35 DAS) followed by second spray at 15 days interval. The disease severity was recorded at 15 days after second spray on five randomly selected plants per plot. In each plant, 5 leaves from lower, middle and upper part of the plant were graded by using 0-9 scale (Mayee and Datar, 1986) and expressed as per cent disease index (PDI). Per cent reduction of disease over control and per cent increase in yield over control were also computed and presented in the Table 1.


 
Disease scoring scale
 
Score       Description
 
0             No symptom of powdery mildew on leaves.
1             Small scattered powdery mildew specks covering                                 1% or less leaf area.
3             Small powdery lesions covering 1-10 % of leaf area.
5             Powdery lesions enlarged covering 11-25 % of leaf area.
7             Powdery lesions coalesce to form big patches                                      covering 26-50 % leaf area.
9             Big powdery patches covering 51 % or more leaf                                  area and defoliation occur.
        Per cent disease index was calculated by using formula given by Wheeler (1969). Green tender pods harvested at different intervals from each plot and pod yield per plot was converted into tonnes per hectare. Finally the cost of production was analyzed in order to find out the most economic treatment of different management practices. Cost and return analysis were done according to the procedure of Kushwah et al., (2017) and Bhupender et al., (2020). The benefit cost ratio (BCR) was calculated as follows:  
For analyzing the experimental data, arcsine angular transformations were made and the data analyzed with ANOVA in randomized completely block design using IBM SPSS statistics 21 to test for significant difference among the treatments. In the study, observed significant differences at 5% level of significance (P value>0.05) for per cent disease index and yield at different treatments (Walter, 1967).

The results of the experiment are presented in Table 1. From the experimental results it was very clear that all the treatments significantly reduced the disease development compared to the unsprayed plot after two sprays in combination. Among the thirteen combinations, two sprays of hexaconazole (T₄) recorded the least PDI (11.75%) with highest yield of 6.20 t ha^-1, per cent disease reduction was upto 62.53 per cent and 138.33 per cent increase yield, which was significantly superior and found on par with two sprays of tebuconazole 50% + trifloxystrobin 25% WG (T5) (14.27 PDI) with an yield of 5.98 t ha^-1. Least disease control was recorded with two sprays of Bacillus subtilis (T₁) (32.74 PDI) which yielded 3.65 t ha^-1 as compared with unsprayed control 2.60 t ha^-1.
       
The cost benefit ratio has been worked out for different spray schedules and presented in the Table 2. The highest B:C ratio was obtained with spray schedule involving two sprays of hexaconazole (3.23) which is followed by two sprays of wettable sulphur (2.89) and two sprays of tebuconazole 50% + trifloxystrobin 25% WG (2.86). However lowest B: C ratio was observed in unsprayed control (1.48).


       
Fungicides still constitute the predominate part of the control measures used against powdery mildew. Use of newer chemicals has become more popular in recent years because of their quick results, especially in absence of resistant varieties. Foliar spray of hexaconazole, tebuconazole 50% + trifloxystrobin 25% WG and wettable sulphur provided 81.77, 77.86 and 65.52 per cent reduction of powdery mildew in cluster bean, respectively. Hexaconazole is sterol inhibiting fungicide effectively managed the powdery mildew disease. The triazole fungicide, hexaconazole at 0.1 per cent effectively managed the powdery mildew disease and it is sterol inhibiting fungicide where ergosterol was essential to the structure of cell wall and its absence causes irreparable damage to the cell wall and fungus dies off (Ramesh et al., 2013). They will also interfere in conidia and haustoria formation (Bademiyya and Ashtaputre, 2019). They change the sterol content and saturation of the polar fatty acids leading to alterations in membrane fluidity and behaviour of membrane bound enzymes. They affect the cytochrome P-450 enzymes the inhibitors of sterol C-14 demethylation (Nene and Thapliyal, 1993).
       
Sangani et al., (2015) studied the effect of different fungicides against powdery mildew on cluster bean under the field condition during two seasons in 2013-2014 and found that wettable sulphur and hexaconazole were effective in managing the disease by recording least disease intensity of 22.42 and 22.86 per cent and higher disease reduction of 65.69 and 65.02 per cent over control, respectively. Raju et al., (2017) conducted an field experiment to know the effect of different fungicides against powdery mildew disease of capsicum (F1 hybrid ‘Indra’) and found that hexaconazole (0.1%) proved to be the best for the management of powdery mildew with minimum per cent disease index (15.86%) and maximum fruit yield (92.15 t/ha) as compared to untreated control (76.33%) with lower fruit yield (45.83 t/ha). Daunde et al., (2018) also reported that foliar application of hexaconazole (0.1%) has resulted in minimum disease severity of 19.94 per cent with 73.44 per cent control of powdery mildew in chilli as compared to untreated control. Gorak (2017) who conducted a field evaluation of fungicides against powdery mildew of chilli and his study revealed that, there was continuous reduction in powdery mildew severity in the treatment hexaconazole (0.1 %) with mean PDI 16.79.
       
Hingole and Kurundkar (2011) also reported that triazoles were most effective in reducing the powdery mildew intensity of chilli (Leveillula taurica). Among them, penconazole, propiconazole, hexaconazole and difenconazole reduced the disease effectively and enhanced the yield. However, treatment with three sprays of hexaconazole (0.1%) recorded higher net returns compared to other treatments. Several workers reported that, hexaconazole and propiconazole were found to be effective in reducing powdery mildew severity in different crops (Naik and Nagaraja, 2000; Pramod Prasad and Dwivedi, 2007; Akhileshwari et al., 2012; Channamma, 2015 and Jahir Basha et al., 2017).

The severity of powdery mildew in cluster bean can be significantly reduced by the foliar spray of hexaconazole or tebuconazole 50% + trifloxystrobin 25% WG @ 0.1 per cent at two times after the initiation of disease symptoms and later at 15 days interval. It also recorded higher grain yield and higher economic return.

None.