Management of Alternaria Leaf Blight in Pigeonpea through Host Plant Resistance and Micronutrient Application

Pigeonpea is the fifth prominent pulse crop in the world and the second most important pulse crop in India. It is grown under diverse agro ecological conditions as it has great potential to tolerate drought. Pigeonpea production is amenable for many biotic constraints. Among these, leaf blight caused by Alternaria alternata found to occur in alarming proportion recently in Tamil Nadu. Pigeonpea leaf blight caused by A. alternata was first reported from Varanasi, India by Pavgi and Singh (1971). Later, Kannaiyan and Nene (1977) reported its occurrences from Hyderabad as a minor disease. Now it is widely prevalent in Andhra Pradesh, Bihar and Tamil Nadu. This disease has been reported to cause yield losses up to 40-50 percent (Kushwaha et al., 2010). Leaf blight of pigeonpea inflicts yield losses ranged from 20-80 percent (Sharma et al., 2013).
 
In Tamil Nadu the incidence of leaf blight ranged from 3- 40 PDI and it is highly favoured by light drizzling coupled with high RH. Managing plant diseases through the use of resistant cultivar is one of the most effective, eco friendly and economical strategy. Micronutrients are capable of  controlling  plant pathogen infection in plants either directly by antagonising the pathogen or indirectly through enhancing plant defence mechanism by systemic acquired resistance and or stimulating antagonist population in rhizosphere (Graham and Webb,1991). Application of nutrients viz., Mn, Cu and B can release Ca from plant cell, which interact with salicylic acid and activate systemic acquired resistance mechanisms (Reuveni et al., 1997; Reuveni and Reuveni, 1998). Fungicides are commonly used for the effective control of foliar diseases. The efficacycarbendazim + mancozeb against Alternaria leaf blight was earlier reported by (Venkatramanamma et al., 2014). Hence, the present study was carried out to identify the resistant sources and to evaluate the efficacy of various micronutrients and the combination fungicide viz carbendazim 12% + mancozeb 63% against the leaf blight in pigeonpea.  

Isolation and maintenance of pigeonpea leaf blight pathogen A. alternata
 
The leaf blight pathogen of pigeonpea (A. alternata) was isolated from the leaves infected with characteristic symptoms. The infected portions were cut into one cm bit and surface sterilized with 0.1 per cent mercuric chloride (Hgcl₂) solution for 30 sec and washed thrice in series of sterile distilled water and transferred to sterilized Petri plate containing Potato Dextrose Agar (PDA) medium amended with a pinch of streptomycin sulphate (Riker and Riker, 1933). The Petri plates were incubated at room temperature (28 ± 2^oC) for 5 days and observed periodically for the growth of the fungus. Isolates were purified by single spore isolation maintained on PDA slopes for further use.
 
Evaluation of pigeonpea genotypes for leaf blight resistance
 
Twenty four genotypes received from AICRP pigeonpea coordinating centres were screened for their resistance against leaf blight consecutively for three years from 2015 -2018 at Pulses Breeding Station, Centre for Plant Breeding and Genetics, Tamil Nadu Agricultural University, Coimbatore. Two local varieties viz., CO5 and CO6 were also included in the screening programme. Seeds of 24 genotypes along with above mentioned two varieties were sown in two rows of 4 meter row length during the first week of August in all the three years. Three replications were maintained for each genotypes and the crop was maintained by following standard package practices as per the recommendation of the Crop Production Guide, TNAU, Coimbatore.  After 30 and 45 days of sowing the plants were sprayed with conidial suspension of A. alternata (5 × 105 conidia/ml) to enhance the inoculum load of the pathogen which make screening the most effective. The observations were recorded on leaf blight incidence by following 0 to 9 scale on 60 days after sowing. The disease grade and resistance scale used for identifying resistant sources for leaf blight is furnished below (Anonymous, 2015).


 
Efficacy of micronutrients and carbendazim 12 %+ mancozeb 63 % against leaf blight incidence
 
Field trials were conducted during kharif 2015-16, 2016- 17 and 2017-18 to find out the effect of micronutrients and carbendazim 12% + mancozeb 63% on the leaf blight incidence of pigeonpea. The experiment comprised of seven treatments and each treatment was replicated thrice and arranged in RBD. The cultivar used was Co Rg7. Foliar spraying with different micronutrients (as presented in Table 3) was done on 30 Days  After Sowing (DAS). Fifteen days after spraying of micronutrients (45 DAS) one fungicidal spraying was given with carbendazim 12%+ mancozeb 63% @ 1g/ lit.  The observations were recorded on the incidence of leaf blight on 60DAS using 0-9 scale and Per cent Disease Index was worked out. The data recorded on grain yield also.
 
Statistical analysis
 
The data were analysed statistically as per Gomez and Gomez (1984).
 

A total 24 pigeonpea genotypes were screened for their resistance against leaf blight consecutively for three years from 2015-2018 under field condition. Among these, four genotypes viz., BDN 2, IPA 8F, IPA 15F and MA 6 were found to be resistant to leaf blight in all the three years of evaluation with the mean disease grade of 2.1, 2.4, 2.6 and 2.4 respectively. Nine genotypes viz., CRG 9701, ICP 7119, ICP 2376, KPL 43, KPL 44, MAL 43, WRGE 65, WRP-1 and ICP 8863 showed moderately resistant reaction with the mean disease grade ranged from 3.2-4.2. The varieties viz., CO5 and CO6 used in this study also showed moderately resistant to the disease with the mean grade of 4.2 and 3.6 respectively. Remaining 11 genotypes viz., BRG 1, BRG 2, BSMR 853,BSMR 736, RVSA 07-31, RVSA 07-29, RVSA 07-10, BRG 3, BRG 4, JKM 189 and MAL 13 exhibited susceptible reaction to leaf blight (Table 1 and 2). Several workers previously evaluated the pigoenpea genotypes for leaf blight resistance and identified resistant sources. Kumar and Rani (2010) screened 96 genotypes of pigeonpea in field under artificially inoculated conditions against leaf blight and found that genotypes viz., RAUP-32, RAUP-34 and Pusa-(B)-35 were resistant, 12 were moderately resistant to the leaf blight. The pigeonpea genotypes, MA-128-1, MA-128-2 and DA-2 were found to be resistant to Alternaria blight under natural field condition (Venkateswarlu, 1981). These entries were also reported to be resistant under artificially inoculated condition in green house (Kannaiyyan and Nene, 1986.). Out of 106 genotypes, seven genotypes viz. Path 402, Path 407, NDA14-4, NDA14-15, NDA-14-16, NDA-14-29 and NDA-14-36 were found resistant, 33 genotypes were moderately resistant, 53 genotypes were moderately susceptible and 13 genotypes were susceptible to Alternaria blight (Rathore et al., 2018).





Field experiments were conducted consecutively for three years from 2015-16 to 2017-18  to evaluate  the efficacy of micro nutrients and the combination fungicide viz., carbendazim 12% + mancozeb 63% on leaf blight incidence. The foliar spraying of micronutrients viz., ZnSO ₄, MnSO ₄, FeSO ₄ CuSO ₄ and Borax @ 0.2 per cent and Na Mo₃ @ 0.1 percent on 30 DAS followed by  spraying of  carbendazim  12% + mancozeb 63% @ 1g / lit on 45 DAS was found to be promising in reducing the leaf blight incidence. Among these, foliar spraying of  MnSO 4 @0.2 % on 30 DAS + carbendazim  12% + mancozeb 63% @ 1g / lit on 45 DAS recorded the lowest leaf blight incidence (8.1 PDI) and  with the highest disease reduction of 76.9% (Table 3). Manganese has been reported to control the diseases viz., blast and Alternaria leaf spot in rice (Simoglou and Dordas, 2006), soybean leaf spot and rust in cereals (Thompson and Huber, 2007). Manganese inhibits the induction of amino peptidase, an enzyme which supplies essential amino acids for fungal growth and pectin methyl esterase, a fungal enzyme that degrades host cell walls. It controls lignin and suberin (lipophilic macromolecule found in plant cell which prevents the fungal invasion) biosynthesis (Vidhyasekaran, 1997) through activation of several enzymes of the shikimic acid and phenylpropanoid pathways (Marschner, 1995). The micronutrient viz., Mn has an important role in photosynthesis and phenol biosynthesis and several other functions (Dordas, 2008). The efficacy of carbendazim + mancozeb against Alternaria leaf blight has been proved by Venkatramanamma et al., (2014).   


 
Foliar spraying of ZnSO ₄ @ 0.2 % + carbendazim 12% + mancozeb 63 %@ 1g / lit ranked next in reducing the leaf spot incidence by recording 9.8 PDI with 72.0 per cent disease reduction. In the control plot leaf spot incidence of 35 PDI was registered. Zinc nutrition plays a pivotal function in plant immune responses (Shirasu et al., 1999) and decreased the plant disease symptoms (Grewal et al., 1996; Li et al., 2016; Machado et al., 2018). Plant nutrients inhibit the disease development by changing the plant physiology or by affecting pathogen or both of them. Plant pathogen dynamics in response to nutrient availability are thought to be more complicated as nutrient deficiency/sufficiency in plant system modulate host physiological system and plant reacts differentially (susceptible or tolerance) against different pathogens. However, some nutrient elements have a direct and greater impact on plant diseases than others (Graham and Webb, 1991).
 
Foliar spraying of MnSO 4 @0.2% on 30 DAS + carbendazim 12% + mancozeb 63% @ 1g / lit on 45 DAS  registered the highest grain yield of 905 kg/ha as against 703 kg/ha in the control (Table 3). Application of MnSo₄ at 2.5 kg ha^-1 increased the yield in soybean (Tomar et al., 1991). The micro nutrients viz., Mn, Zn, B and Mb are important for increasing the productivity of soybean (Devarajan and Palaniappan, 1995). It was followed by ZnSO 4 @ 0.2 % on 30 DAS + carbendazim 12% + mancozeb 63% on 45 DAS which recorded the grain yield of 885 kg/ha. This was in concordance with findings of Yashona et al., (2018) who found that foliar application Zn enhances the growth and yield attributes of pigeonpea.

In the present study the pigeonpea genotypes viz., BDN 2, IPA 8F, IPA 15F and MA 6 were identified as resistant sources  for pigeonpea leaf blight caused by A. alternata  which will be further used for breeding programme. The micronutrient viz., manganese and the fungicide carbendazim 12%+ mancozeb 63%, effective against leaf blight should be one of the component for sustainable integrated disease management strategy.

The authors gratefully acknowledge the funding from the All India Co-ordinated Research Project on pigeonpea for carrying out this work. The authors also thankfully acknowledge the Department of Plant Pathology, Tamil Nadu Agricultural University, Coimbatore, India for providing laboratory facilities for carrying out the research work.

All authors declare that they have no conflicts of interest.