In this article we will discuss about:- 1. Introduction to Maize Pathogen 2. Epidemiology of Maize Pathogen 3. Environmental Factors 4. Disease Management 5. Disease Resistance.
Introduction to Maize Pathogen:
The disease banded leaf and sheath blight incited by Rhizoctonia solani f. sp. Sasakii Exner (Thanetophorus sasakii (Shirai) Tu & Kimbro), is found in most parts of the world and is capable of attacking a wide range of host plants including maize, causing seed decay, damping off, stem canker, root rots, aerial blight and seed decay.
Detailed work on maize pathogen suggests need for critically determining the correct taxonomy of the pathogen. The teleomorphic stage of maize pathogen has not been recorded in nature or in vitro so far, whereas it has been observed on rice.
Pathogenicity tests revealed that R. solani AG-1-1A was the most strongly pathogenic to leaf sheaths and stalks of maize pathogen, followed by R. solani AG-2-2. Differences were observed in the susceptibility of four maize cultivars to the anastomosis groups of R. solani and R. zeae.
Epidemiology of Maize Pathogen:
The rice pathogen has wide host range and infects plants belonging to over 32 families in 188 genera. R. solani f.sp. sasakii infects by artificial inoculations a number of crop plants belonging to families Gramineae, Papilionaceae and Solanaceae.
Maize pathogen has also been found to be infected by isolates from rice, sugarcane, arrow root and some grasses. Cross inoculation tests indicated that rice and turmeric isolates did not differ either in pathogenicity or morphological characters.
The primary source of inoculum are the sclerotia in soil or in infected host debris, and the active mycelium on the other grass hosts that grow in the vicinity of maize plant in the field. The viability of sclerotia is greatly influenced, to a large extent, by environmental and soil conditions. The germinated sclerotia come in touch with lower leaf sheath or drooping leaves contacting soil.
The infection progresses upward in bands killing the tissues in advance. The infection extends to distal half of the leaf lamina exhibiting buff-white mycelial growth in patches on the affected parts within 48-72 h of infection. The primary inoculum becomes more active during the period of pre-flowering stage (30 to 40-day old plants).
Secondary spread is due to contact of healthy plants with infected leaf or sheath or vice-versa. The infection continues to mid-dough stage infecting the entire plant including the ear except tassel. These seeds are not considered to be source of inoculum and may not play a major role in severe disease outbreaks.
If cob infection occurs at the late stage, the germination of seed is adversely affected resulting into seed rot and seedling blight. The sclerotia formed on the infected plant organs fall down on soil surface get, buried in soil with infected host debris and serve as a source of primary inoculum for maize and other host crops in field.
Environmental Factors of Maize Pathogen:
The disease is favored by warm and humid climate. The optimum temperature for in vitro growth of the pathogen is 30°C and the highest level of disease is induced when RH is in the range of 90-100 per cent. Below 70 per cent the disease development was negligible. Maximum relative humidity and rainfall largely influence the spread of the disease.
Studies on the meteorological factors in relation to disease progress revealed that optimum temperatures (near about 28°C) were essential for infection, disease development and subsequent spread of the maize pathogen. Rainfall over 100 mm in the first two weeks of inoculation favored early infection and disease development. .
Positive correlation was noticed between rainfall and disease progress. This led to obtain partial regression coefficients to derive a simulation equation based on meteorological factors for the prediction of disease. However, it needs more data to standardize the model for practical utility.
Disease Management of Maize Pathogen:
1. Fungicides like carbendazim, benodanil, validamycin, topsin M, Rhizolex etc. were effective in reducing disease severity under field conditions.
2. Benodanil (150 p.g/ml a.i), Validamycin and Carbendazim reduced disease by 33.8, 33.5 and 23.7 percent, respectively.
3. Bavistin 50 WP Benomyl 50 WP and Brestan gave 87, 82 and 77 % disease control, respectively and resulted in significant increase in grain yield. Three foliar applications of Propiconazole at 10-day intervals exhibited reduction in disease severity and resulted in higher grain yields.
Through Culture Practices:
1. Inter-cropping of maize with legumes specially with soybean effectively reduced the activity of the pathogen in soil.
2. Maintaining the plant population at an appropriate level and applying cattle compost before planting decreased the disease level and its subsequent spread in field.
3. mechanical stripping of the second and third leaf-sheath from the ground level at the age of 35-40 day old crop or knee high stage which is effective in checking disease development further up.
Through Biocontrol Agents:
Biological control of disease offers the preffered mode of environmental friendly control of disease in maize pathogen including banded leaf and sheath blight. Application of Pseudomonas fluorescens has been reported to control the disease in the field conditions besides improving the plant growth.
The biocontrol agent used, exhibited the production of volatile ammonia and HCN under in vitro conditions Sivakumar (2000) demonstrated a very effective control by seed treatment of peat based formulation at the rate 16 g/kg, or soil application at the rate 2.5 kg/ha, or spraying liquid formulation twice at the rate 5 g/litre of water.
Manisha (2002) also observed that combined seed treatment and foliar treatment with Pseudomonas fluorescens from maize rhizosphere was most effective in giving 30% reduction in disease incidence of banded leaf and sheath blight. Similarly, Meena 2003 demonstrated that among Trichoderma, T. harzianum has shown good promise against the maize pathogen both in vitro and in vivo conditions.
Disease Resistance of Maize Pathogen:
A variety of maize pathogen materials including -inbred lines, single crosses, double crosses, double top crosses etc., were evaluated in the Indian maize programme. Among the inbred lines C (coordinated) M (maize) 104 (yellow-flint), and CM 300 (white flint) were determined to be the elite lines in terms of resistance.
In China, 844-1, SZ-I, 884-2, MS08, 8S4-1, WP- 4, WP-5, H138, H158, KI 1414 and several other materials were rated as resistant during 1994-95 crop season. Maize varieties Jinguok, Suweon 83, Suweon 87, Suweon 89, P .3055, P .3160, DK 689 and XCG 51 showed high tolerance levels to BLSB in Korea.
A number of CML lines from CIMMYT and other materials were evaluated in India and China and many lines were identified having reasonable level of resistance. Line CML- 1 in particular was found to be elite one both at Delhi and Pantnagar in India. Inbred lines CM 104, CM 103, CM 300, CM 105, P 217407, CM 600 and hybrid VL 43 have been found resistant under field and laboratory evaluations.
Of these, CM J.04 and 105 have been reported earlier to be resistant source to banded leaf and sheath blight of maize. A total of 3000 genotypes have been screened. In India under ICAR- CIMMYT Collaborative Activity, out of them 50 materials have shown some level of tolerance to BLSB and these materials are being advanced to upgrade their level of resistance by adopting cyclic breeding approach.
Ahuja and Payak (1978, 82, 86) found under laboratory tests that the growth of the pathogen was completely inhibited by carbendazim, benodanil, thiobendazol and validamycin while field condition benodanil, validamycin and carbendazim were most effective in reducing disease severity.
Lai (1985) observed that foliar spray with thiobendazole (60 W.P. at the rate 0.05 per cent followed by carboxin (75 W.P. at the rate 0.1 per cent) and Duter (20 W.P. at the rate 0.05 per cent) were most effective in reducing disease.
Sharma and Hembram (1990) have worked out a simple and economical method to control this disease. The methodology comprised of mechanical stripping of third leaf sheath from the ground level which is effective in checking disease developing further up. This method was also found to be effective in minimizing the yield loss.