Effectiveness of nutrient administration technique in managing late leaf spot disease of groundnut

Introduction

Groundnut (Arachis hypogaea L) is an important grain legume grown for oil. Among different abiotic stresses, nutrient deficiency is causing reduced oil quality and yield while among abiotic stresses, foliar diseases cause severe yield loss. Of different foliar diseases, the late leaf spot (LLS) disease, caused by a fungal pathogen, Phaeoisariopsis personata (Berk. & Curt) Arx)). together with early leaf spot (ELS) (Cercospora arachidicola (Hori)), both these foliar diseases can result in yield losses up to 70% [1] . These major biotic and abiotic stresses are often having vital relationship in nature where deficiency of nutrients succumb groundnut plants to late leaf spot disease infection [2,3] . Both ELS and LLS can be managed through application of beneficial microbes [4] , cultural methods such as crop rotations [5] , farmers are heavily relying on application of chemical fungicides [6] . Tebuconazole is an effective systemic fungicide that reduced LLS intensity to a tune of 52% [7] . Recently, it was reported that application of tebuconazole 50%+ trifloxystrobin 25% WG @ 0.25% was highly effective against both ELS and LLS [8] . However, indiscriminate usage of fungicides in groundnut ecosystem can lead to development of resistance in pathogens to commonly used fungicides. Moreover, chemical fungicidal application can lead to soil, water pollution and can cause potential harmful effects to environment. Owing to the relationship existing between nutrient host and pathogen, mineral nutrient supplements are being used in various crops for managing several diseases. Both soil borne and foliar diseases are managed effectively using nutrient supplements in groundnuT 2 [ , 9]. In addition, mineral nutrition will also aid in increasing yields. In a study in groundnut, foliar sprays with NPK-19:19:19 @ 0.5%; KNO3 @ 0.5% and KCl @ 1% at 60 DAS have recorded significantly highest pod yield (2335 kg ha [-1]), followed by KNO3 (2190 kg ha [-1]) and KCl @ 1% (2154 kg ha [-1]) [10] . Indeed, the balanced nutrition has been the first line of defense in plant protection due to direct involvement of mineral nutrients [11] . Supplementing various plant nutrients in enhancing plant health can be due to direct action on pathogen control, plant growth and development, and by imparting disease resistance [12] . With respect to groundnut late leaf spot disease, application of nutrients Magnesium, Copper, Sulphur and Potassium has significant impact in reducing disease incidence by imparting resistance via antioxidant enzyme activities [2,13] .

In our present study, we have proposed to evaluate the effect of method of application of these mineral nutrients on LLS disease management in groundnut. Mineral nutrients were applied to groundnut plants by soil application and foliar spray in sand culture to note the efficiency of method of application.

Materials and Methods

Sand culture

Sand culture technique was used to grow groundnut plants using Hoagland solution. For the experiments in the present study, Sand culture using 1.00 mm quartz sand with Hoagland nutrient solution (one third volume of sand) supplied at weekly intervals with precautionary measures to avoid algal growth was taken up for good plant growth.

Source of groundnut cultivar, K-6:

The popular groundnut cultivar (CV), K-6 that is highly susceptible to LLS and rust diseases was used in the present study. The K6 seeds were procured from Agricultural Research Station, Kadiri of Acharya NG Ranga Agricultural University, Ananthapuramu district of Andhra Pradesh, India. The K-6 is a high yielding, drought tolerant variety with light tan kernels. This CV matures in 100-105 days with 70% shelling outturn and a kernel weight of 39 g sound mature 100-kernel weight.

Efficacy of method of nutrient application on groundnut late leaf spot disease development

Susceptible groundnut cultivar (K-6) was grown in sand culture with nutrients applied at weekly intervals. The complete Hoagland and Arnon (1950) nutrient solution was served as check. Additional nutrients, 0.5% KNO 3 , 0.5% MgSO 4 and 0.2% CuSO 4 were assessed for their efficacy in reducing disease development when applied as foliar spray in comparison to soil administration Tebuconazole treated seed grown in nutrient solution served as fungicidal check. Three replications were maintained for each treatment.

Inoculation of Phaeoisariopsis personata

Late leaf spot (LLS) diseased leaves were collected from field at 70 DAS. Conidia of Phaeoisariopsis personata were collected into sterile distilled water under aseptic conditions. Conidial concentrations were determined with a haemocytometer. Conidial load in suspension was adjusted to 1x10 [6] conidia/ml and was sprayed on to hydroponically grown groundnut cultivars at 20DAS and 35 DAS. Plants were covered with transparent plastic sheets to maintain suitable conditions for disease development.

Assessment of disease

Disease development was monitored starting from first day of inoculation. The following parameters were studied for the components of resistance and data was taken from three replications a. The incubation period (IP), defined as days from inoculation to appearance of the first lesion, was recorded on each leaf every day from 2 DAI to 21 DAI. b. Latent period (LP), defined as days from inoculation to the appearance of the first sporulating lesion, was recorded on each leaf every alternate day from (1975) [14] .

Where, S i = Disease incidence at i [th] day of evaluation k =Number of successive evaluation of the disease d =Interval between i and i-1 evaluation of the disease

Results and Discussion

Efficacy of method of nutrient application on groundnut late leaf spot disease development.

Incubation period and latent period: After first inoculation, incubation period was significantly highest in fungicidal check (13.67 days) and CuSO 4 either applied in soil or as foliar spray (13.67 days) and MgSO 4 foliar spray (13.33 days). All other treatments i.e., KNO 3 foliar spray (10.33 days), MgSO 4 soil application (10.00 days) and KNO 3 soil application (10.00 days) were on par with control (10.00 days) indicating lack of effect of these nutrients on incubation period after first inoculation.

(Table 1) However, after second inoculation at 35 DAS, in control i.e., 100 per cent nutrient supplements, incubation period was 4 days and was on par with all other nutrient supplements except CuSO 4 soil application (6.00 days) which had maximum incubation period followed by tebuconazole seed treatment (6.00 days). Other nutrient treatments showed incubation period on par with 100 per cent nutrient supplied plants (control). Further, least incubation period was observed in KNO 3 soil application which was on par with its foliar spray. Efficacy of CuSO 4 soil application not only prolonged incubation period but was also on par fungicidal check which showed significantly highest incubation period (6.00 days) indicating higher efficacy of CuSO 4 so when absorbed through roots specially to prevent subsequent disease development. 1). It may be observed here that a general decrease in total incubation and latent periods up on second inoculation compared to first inoculation. In the present study, fungicidal check and CuSO 4 soil application extend the incubation and latent periods significantly compared to control and other nutrient supplements.

Lesion number:

Lesion number observed at 10, 16 and 20 days after first inoculation, showed significant differences among treatments. At 10 DAI, significantly highest lesion number was observed in control (1.33/ leaf) and was on par with KNO 3 soil application and foliar spray and MgSO 4 foliar spray (1.00/ leaf) while no lesions were observed when CuSO 4 applied either to soil or foliar spray, MgSO 4 applied to soil and fungicidal check in which incubation period was more. ( When total lesion number was considered, control and KNO 3 foliar spray showed highest lesion number (10.33/leaf) followed by KNO 3 soil application (6.00/ leaf), MgSO 4 soil application (5.33/leaf), MgSO 4 foliar spray (4.00/ leaf), CuSO 4 soil application (3.33/leaf) which showed intermediary values. Lowest lesion number was observed in fungicidal check (2.00/leaf) which was on par with CuSO 4 foliar spray (2.67/leaf). Per cent decrease in lesion number was highest in fungicidal check (49.03%). Among nutrient supplemented treatments, CuSO 4 foliar spray reduced 43.23 per cent followed by CuSO 4 soil application (38.24%). MgSO 4 foliar spray reduced lesion number by 33.78 per cent while soil application reduced by 25.28 per cent. KNO 3 foliar spray was similar to control while soil application reduced lesion number by 21.39 per cent. Thus, CuSO 4 and KNO 3 were more effective when absorbed through roots while MgSO4 when applied as foliar spray in reducing lesion number.

Lesion diameter:

Lesion diameter observed at 12, 16 and 20 days after first inoculation showed significant differences among treatments (Table 3). At 12 DAI, lesion diameter was significantly higher in control and was on par with KNO 3 foliar spray (0.27mm) and MgSO 4 soil application (0.20 mm). Fungicidal check showed least lesion diameter which was on par with CuSO 4 soil application (0.13mm) and CuSO 4 foliar spray (0.12mm). At 16 DAI, maximum lesion diameter was observed in control (1.77mm) which was on par with KNO 3 soil application and foliar spray, and CuSO 4 soil application (1.70mm). CuSO 4 foliar spray showed smallest lesion diameter (0.67mm) which was on par with fungicidal check, MgSO 4 soil application and MgSO 4 foliar spray (1.00mm). At 20 DAI, significantly highest lesion diameter was observed in control (4.30mm) compared to all other treatments while least lesion diameter was observed in fungicidal check (0.43 mm). All the nutrient supplemented treatments showed significantly lesser lesion diameter than control yet significantly more lesion diameter than fungicidal check. CuSO 4 treated plants either through roots or foliar showed 30 per cent reduction in lesion diameter compared to control while MgSO 4 applied treatments reduced 17.21 and 14.60 per cent in foliar spray and soil application respectively. KNO 3 application in contrast with lesion number reduced 12-13 per cent reduction in lesion diameter. Fungicidal check offered about 48.02 per cent reduction in lesion diameter compared to control. After second inoculation at 35 DAS, observations at 4 DAI showed longest lesion diameter in control (0.32mm) which was on par with KNO 3 foliar spray (0.23mm), fungicidal check and CuSO 4 soil application (0.20 mm). CuSO 4 foliar spray (0.12mm) showed significantly shortest lesion diameter and was on par with MgSO 4 foliar spray (0.20mm), MgSO 4 soil application (0.17mm) and KNO 3 soil application (0.20mm). At 8 DAI, significantly longest lesion diameter was observed in control (2.50mm) while shortest lesion diameter was observed in fungicidal check (1.00mm). Nutrient supplementation showed intermediary values which were significantly higher than fungicidal check yet lower than control. Fungicidal check offered 24.40 per cent reduction in lesion diameter when compared to control. Foliar spray of CuSO 4 and MgSO 4 aided in reducing lesion diameter by 14.38 and 14.94 per cent respectively followed by MgSO 4 soil application (12.75%). KNO 3 foliar spray reduced lesion diameter by 10.59 per cent in contrast with lesion number where reduction was not observed. KNO 3 soil application and CuSO 4 soil application aided in reducing lesion diameter by 8.98 and 7.42 per cent respectively. With increase in age of the crop and upon subsequent inoculations, foliar application proved better in reducing lesion diameter when compared to soil application while at earlier stages both had similar effect on reducing lesion diameter. In many cases aerial spray of nutrients is preferred and gives quicker and better results than the soil application [15] probably due to increased nutrient use efficiency than that through soil application [16] . KNO 3 supplemented either as foliar spray or to soil, neither able to increase incubation and latent periods nor decreased number of lesions and AUDPC. K and N when applied as foliar spray resulted in amount of disease as similar to that in control. This might be due to N content predisposes the plants to pathogen especially when balanced with K [17] . Further foliar sprays of nutrients such as phosphates, K and N were found to induce disease. In the present study, MgSO 4 administered through roots as well as leaves had similar impact in reducing disease but with increase in age and subsequent inoculations, foliar spray proved to be better in tolerating pathogen infection. MgSO 4 foliar spray though didn't increase incubation period up on subsequent inoculation, it considerably reduced lesion number and AUDPC. CuSO 4 when applied to soil prolonged incubation and latent periods but probable lack of inhibition in spore germination might have resulted in more lesion number. On the other hand, CuSO 4 when applied as foliar spray reduced lesion number owing to inhibition of spore germination. Application of Cu as foliar spray reduced disease to greater extent which might be also due to its effect in activating SIR. Trace elements can act on plant defense mechanisms triggered against pathogen infection. Mn and Cu might act as cofactors of metalloprotein in antioxidant enzymes, for which Mn ions serve as an inducing agenT 12 [ , 18]. Cu and B can increase the intracellular Ca [2+] cations, and interact with SA and activate SIR [19] which reduced fungal disease in MR219 rice cultivar with increased rice yield [20] . Foliar spray with H 3 BO 3 , CuSO 4 , MnCl 2 or KMnO 4 separately induced systemic protection against powdery mildew in cucumber plants [21,22]

Conclusion

Groundnut late leaf spot disease can be managed by administering mineral nutrition. Among soil application and foliar application of potential nutrients, CuSO 4 Foliar spray is effective in disease management by prolonging incubation period and latent period, reduced lesion number with small lesion diameter and least AUDPC followed by MgSO 4 foliar spray and soil application of CuSO 4 and MgSO 4 which are on par with fungicidal check. Second inoculation at later stages of crop reveled that foliar spray of CuSO 4 followed by MgSO 4 are effective in reducing disease . Hence, supplementation of CuSO 4 and MgSO 4 were turned effective in reducing disease when applied to soil during initial stages and as foliar spray with increase in age of the crop.

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