Why is it a problem? First detected in Bhutan in 2006, this disease quickly became the most serious threat to maize production. A major research and development program successfully resulted in the release of GLS tolerant maize varieties. This disease can now be managed relatively easily.
Where and when is it a problem? The presence of this disease was first confirmed in Bhutan in 2007. It is mainly a problem above 1800 m asl. It reached epidemic status in 2007 when it affected 4,193 maize growing households and a total area of 4,822 acres. Production loss to the disease was estimated at 6,504 Mt, with between 50% and 70% of farmers affected.
Symptoms first occur on the low leaves of the maize plant. These slowly move upwards. Lesions begin as small (pin-point), regular, elongated brown-gray necrotic spots growing roughly parallel to veins. These spots usually have yellow or chlorotic borders (a halo). They are more easily observed when the leaf is held up to the light. Severe symptoms lead to the entire senescence of leaves and poor grain filling. Plants infected at or before tasseling appear as if they are blanched by hot boiling water. The entire maize field gives a brown appearance.
Confusion with other pests: Disease symptoms differ from turcicum leaf blight which also commonly occurs on maize. Sometimes both GLS and TLB can occur in the same plant which can cause straw-like blighting of plants.
Lifecycle: As temperatures rise and humidity increases the spores in crop residues start to develop. Once mature they are picked up by the wind and infect the lower leaves of the new maize crop. Infection requires the leaf surface to be wet for 11-13 hours and the relative humidity on the leaf canopy to be above 90%.
Dispersal: The main source of inoculum is the infected maize crop residues, especially the leaves and the sheath left on the soil surface. Once developed the spores in these residues are blown by the wind.
When can damage be expected? Disease development is favoured by prolonged periods of leaf wetness and high humidity. GLS and TLB outbreaks in Bhutan were also attributable to the predominant farming practices at the time:
(1) mono-cropping of maize with occasional rotation with potato;
(2) use of maize stalks and crop residues as livestock bedding, and subsequently as FYM (farm yard manure)—a major source of nutrients for the crop; and
(3) reduced or minimum tillage due to steep terrain so residues are not deeply incorporated into the soil.
Hosts: In Bhutan it has only been reported from maize.
Gray leaf spot (GLS) is only a problem above 1800 m asl. It can be controlled by adopting good cultural practices and utilizing GLS tolerant maize varieties that are now readily available in Bhutan.
Early detection is important to prevent major losses. Regular monitoring will detect the disease in time. Monitor the field at least every four weeks (fortnightly if wet and warm conditions are prolonged), from whorl through to dent stage.
Effect of variety
GLS resistant varieties are now widely available and used in Bhutan. In 2011, two new genotypes which are tolerant to GLS and TLB were released, named as Chaskarpa for genotype ICAV305 (Entry No. 38) and Shafangma Ashom for genotype S03TLYQ AB05 (Entry No. 35). With this release and rapid seed increase through the Community Based Seed Producers (CBSP) groups, 75% seed replacement of the affected farmers with the two GLS tolerant varieties has been accomplished by the 2013 planting season. Some pre-existing varieties also showed some tolerance. These include Yangtsepa, Khangma Ashom 1 and Khangma Ashom 2.
- Select appropriate GLS tolerant varieties. Chaskarpa or Shafangma Ashom will need to be used in susceptible areas.
- Provide proper nutrients during growth. Don’t over apply nitrogen.
- After harvest collect infected crop residues and either burn them or bury them through deep tillage.
- Follow crop rotations such as maize-potato, maize-upland rice, maize-soybeans and maize-vegetables
Chemical control should no longer be necessary to control this plant now that tolerant varieties are available, and is unlikely to be economically justified. Contact NPPC if there is a serious outbreak.
Katwal et al. 2013 Evaluation of Gray Leaf Spot Tolerant Genotypes from CIMMYT in the Highland Maize Production Eco-systems of Bhutan- Katwal et al. 2013 (Maize disease Bhutan)
Image acknowledgements: D.MALVICK