Common Maize Diseases in Nepal

Maize (Zea mays) is an annual grass in the family Gramineae. Maize ranks second after rice in area and production. Maize cultivation is a way of life for most farmers in the hills1 of Nepal. It is a traditional crop cultivated as food, feed and fodder on sloping Bari land (rainfed upland) in the hills. It is grown under rainfed conditions during the summer (April-August) as a single crop or related with millet later in the season.

There are many diseases in maize which affect the productivity of maize. Some are mild and some are severe. Some common diseases in maize are:

• Gray leaf spot.
• Ear rot in maize.
• Northern corn leaf blight.
• Southern corn leaf blight.
• Banded leaf and sheath blight.

Diseases, their causes, symptoms as well as their preventive measures are given below-

Grey leaf spot:
Its occurrence was recorded in the Kavrepalanchowk district of Nepal. Grain yield losses of maize of up to 19 percent and 18 percent, respectively, on the local variety and on the improved OPV (Ganesh -1) were observed in field experiments (Manandhar and Baidya, 2010). The severity of GLS was high in the high hill region, adversely affecting farmers who live with limited resources. The disease has been observed spreading over the years in 17 districts in the eastern, central and mid-western regions of the country. GLS is evident on plants as small spots first on lower leaves of plants at tassel initiation. The disease moves upwards and spots change into long characteristics lesions within a month turning plants into a diseased field. The disease is significant since it rapidly destroys foliage when the plant is near at grain maturity

Caused by: caused by Fungus Cercospora Zea-Mayalis.
Symptoms: includes leaf lesions, discoloration(chlorosis), and foliar blight.
Prevention and treatment: prevention of grey leaf spot includes providing crops better airflow, sufficient sunlight and soil drainage. Providing proper nutrients and avoiding overcrowding are the ways to go. Spraying Foliar fungicides will help treating grey leaf spot.

Ear rot in Maize:

A number of pathogens have been observed in Nepal. Among the various ear rotting organisms, Fusarium moniliforme Schw. was found to the most common and most destructive to the crop. Pathogens like Diplodia maydis (Berk) Sacc. and Rhizoctonia zeae Voorhees were also common in the mountains but the infection was very low. Although Fusarium ear rot is distributed throughout the country, it is most prevalent in the mountains with cool and humid weather conditions. In these areas, the crop stands for 6-8 months in the field and the high rainfall provides favorable conditions for disease incidence. The infection rate in the mountain is usually 30-40% whereas in the inner terai and the Kathmandu valley it is only about 5-10%. In some varieties an intensity of infection of 80-90% has been reported. Such intensity of infection results in very high losses in grain yield. The improved varieties of maize seem to be more susceptible to this disease than some of the local varieties

Caused by: Aspergillus Flavus.
Ear rot can be observed after hot, dry weather during the latter half of the growing season after pollination.

Symptoms: symptoms are a white to pink or salmon colored, cottony mold that occurs on single or multiple kernels scattered or clustered on the ear.
Decay often begins with insect damaged kernels. Infected kernels are frequently tan or brown or have white streaks.

Prevention and Treatment: To combat ear rot diseases, we need to be sure to clean up or dig any crop residue. Also, rotation of crop which will allow the corn detritus to break down and reduces the presence of the corn.

Northern Corn Leaf Blight:
Northern corn leaf blight (NCLB) or Turcium leaf blight (TLB) is also a foliar disease of corn. The disease is widely distributed throughout the country; however, it is more prevalent in the cool and humid conditions of the foothills. It was not considered as the major disease of maize crop in Nepal until 1985. It is also common in the winter maize of the terai region. The disease usually 34 appears before tasseling and became the most important disease at present. Maize late sown in the season is most affected by this disease in hills. The lesions of northern leaf blight can be observed together with other diseases on same or on different leaves of the plants.

Caused by: Exserohilium Turcium, The anamorph of the ascomycete setospharia turcica.
Symptoms: Typical symptoms of NCLB are canoe-shaped lesions 1 inch to 6 inch long. The lesions are initially bordered by grey-green margins. They eventually turn tan colored and may contain dark areas of fungal sporulation.
Prevention and Treatment: Resistant hybrids, crop rotation, and fungicides are the way to treat NCLB.

Southern corn leaf blight:

This disease is most common in the Terai region but is also found in some pockets of the mountains and the valleys. The disease usually appears at the time of tasseling. The perfect stage Cochliobolus heterosporus Dreschst is also recorded from Chitwan valley.

Caused by: SCLB is a fungal disease of maize caused by the plant pathogen Bipolaris maydis. There are two races of the pathogen. Race O normally attacks only leaves.
Symptoms: Lesions are tan, somewhat rectangular in shape and have reddish-brown margins.
Prevention and Treatment: Foliar fungicides maybe applied early in the growing seasons to corn seedlings as a risk management tool for southern corn leaf blight and other corn diseases, including anthracnose leaf blight and corn eyespot.

Banded leaf and sheath blight:

BSLB is a serious problem to maize production in many hot and humid environments in the tropics and subtropics. The disease was first noted in 1977, and its incidence has been increasing in Nepal. Under natural conditions, disease appears at pre-flowering stage on 30 to 40 days old plants, but infection can also occur on young plants. The disease appears on basal leaf sheaths as water soaked, straw colored, irregular to roundish spot on both the surfaces.

The disease after its occurrence causes direct loss due to premature death, stalk breakage, destruction of leaves, leaf sheaths and ear rot. Banded leaf and sheath blight of maize is known under various names, viz; sclerotia disease, sharp eye spot, oriental leaf and sheath blight, Rhizoctonia ear rot, sheath rot and corn sheath blight etc. It is a destructive plant pathogen with an almost unlimited host range. It aggressively colonizes organic debris and thus has enough saprophytic survival ability. No sexual spores are formed and only sclerotia formed as soil borne propagules.

Caused by: Fungus (Rhizoctonia solani f.sp. Sasaki)

Symptoms: The disease develops on leaves, sheaths, and stalks and spread to the ears.
Leaves and sheaths of corn develop a characteristic symptom of concentric bands and rings that are discolored, brown, tan or grey in color.
Ear rot is characterized by light brown, cotton mycelium on the ear and the presence of small, round, black sclerotia is usually observed after some time.

Prevention and Treatment:
To reduce incidence and severity of the disease, maize seeds can be sterilized for 10 minutes in 1% sodium hypochlorite solution and 5% ethanol, washed three times with water and dried. An added treatment with formulations containing bacillus subtills enhances this effect.

All the above-mentioned diseases are destructive to the maize production in Nepal or worldwide due to the fact that they occur widespread in maize producing areas. It has been noted that maize diseases reviewed above results in severe economic losses and serves as a potential risk for humans and animals. Therefore, these all need to be managed for improvement in the production of maize crop. Exploration and proper disease identification will be important to help to understand more about the diseases prior the intervention. Variability within pathogen should be considered for screening and breeding for resistance, or while testing sensitivity of the pathogen towards 46 different chemicals. An integrated approach using agronomic, nutritive, or chemical controls should be adopted for an effective disease management. Development of resistant varieties using conventional as well as biotechnological methods will help in controlling these menacing diseases which are still challenges even after several years of their discovery. Studies on epidemiology, diagnosis, yield loss and management of maize diseases (other than host resistance) seem to be quite behind and have to be focused. This review would be helpful to future on maize pathological research works in Nepal.

Some prevention terminologies used above:

1.Crop rotation:
Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. It reduces reliance on one set of nutrients, pest and weed pressure, and the probability of developing resistant pest and weeds. If you grow the same crop in the same place year after year you will get a buildup of pests and diseases specific to that crop. Different crops take different levels of nutrients from the soil and inevitably these become unbalanced, exhausting one nutrient but leaving a lot of another.
Compared to other crops, corn needs lots of nutrients, especially nitrogen. This makes soybeans a good crop to alternate with corn, because soybeans have nodules on their roots that host bacteria that fix atmospheric nitrogen.

2.Foliar fungicides:

Fungicides used to treat leaf of the maize or any other crops is known as foliar fungicides. The best chance that a fungicide treatment will result in a net economic gain for corn or soybean occurs when disease conditions exist which justify making an application. That is, fungicides often result in higher yields when there is enough disease to cause significant yield reductions, such as when a susceptible host is grown in an environment highly favorable to disease development. In fact, research has shown that for corn, the probability of achieving an economic yield increase from a fungicide application in the absence of significant visible disease is usually no better than flipping a coin. Furthermore, in corn, few corn hybrids consistently provide a significant yield benefit from one year to the next when disease pressure is low. Thus, deciding which fields have significant disease risk can help a producer allocate fungicide applications to fields most likely to benefit from such an application.

The principal diseases that might justify a fungicide treatment in some corn fields are gray leaf spot and northern leaf blight. Both of these are caused by fungi that overwinter in corn residues of leaf blades and sheaths, so they are naturally more severe when corn follows corn under conservation tillage. In most fields, however, a combination of rotation and selection of a hybrid with moderate to high resistance (when available) should help keep these diseases from causing damaging yield losses, without applying a fungicide.

Why these diseases must be prevented?

Maize cultivation is a way of life for most farmers in the hills of Nepal. It is a traditional crop cultivated as food, feed and fodder on slopping Bari land (rainfed upland) in the hills.

In 1997/1998, maize was grown on about 800,000 ha which represent 25% of the total area planted to cereals in Nepal. In the same period, 1,367,000 tons of maize were produced, representing about 21% of Nepal’s total cereal production. The proportion of maize area to total cereals was 30% in the high hills, 40% in the midhills and about 11% in the terai2. Maize production as a proportion of total cereal production was 33% for the high hills, 39% for the midhills and 9% for the terai. More than two thirds of the maize produced in the midhills and high hills is used for direct human consumption at the farm level and the ratio of human consumption to total production is higher in less accessible areas.

In the terai, less than 50% of the maize is used for human consumption and a significant part of the production goes to the market. Maize yields fluctuate seasonally and annually especially in the hills. Although maize yields increased slightly over the past five years, there has been very little yield improvement when compared to nationwide yields 30 years ago. This is probably due to the expansion of maize cultivation into less suitable terrain, declining soil fertility, and the sluggish adoption of improved management practices. While productivity in the country is almost stagnant, the overall demand for maize—driven by increased demand for human consumption and livestock feed— is expected to grow by 4% to 6 % per year over the next 20 years. Thus, Nepal will have to resort to maize imports in the future if productivity is not increased substantially.