Exserohilum turcicum

Crop: Maize

Why is it a problem? This was the most important disease of maize, causing significant yield losses. However, the release of tolerant varieties means that it is now relatively easily managed.

Where and when is it a problem?  Turicum leaf blight is most prevalent between 1200 and 1800 masl, co-occurring with GLS between 1700 and 1800 m asl. TLB severity is generally less than that which was observed for gray leaf spot prior to the introduction of tolerant varieties.

SYMPTOMS

Lesions first appear on lower leaves as small water-soaked spots. These spots grow into elliptical brownish-gray lesions with pointed ends. The disease progresses upwards infecting upper leaves.

Confusion with other pests. Disease symptoms differ from GLS which also commonly occurs on maize. Symptoms can also be confused with bacterial wilt, but TLB produces lesions with definite shapes. Sometimes both TLB and GLS can occur in the same plant which can cause straw-like blighting of plants.

BIOLOGY

Lifecycle: Fungal growth and spores survive on plant remains such as leaves, stalks and husks. Thick walled spores allow survival of the fungus for up to 2 years. Survival also occurs on “volunteer” plants. It does not appear to be seed-borne in maize. Water covering the leaves for 6-18 hours, and temperatures between 18 and 27°C, are needed for spore germination and infection.

Dispersal: Spores can be spread to new plants by rain splash as well as in air currents.

When can damage be expected? Heavy dew, frequent light showers, high humidity and moderate temperatures favour the disease. It is more prevalent where reduced tillage methods are employed.

Hosts: In Bhutan, it is a pest in maize. Elsewhere it also utilizes sorghum and wild grasses.

MANAGEMENT PRACTICES

TLB is only a problem between 1200 and 1800 m asl. It can be controlled by adopting good cultural practices and utilizing TLB tolerant maize varieties that are now readily available in Bhutan.

Monitoring

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

TLB 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 new 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.

Non-chemical management

• Select appropriate GLS tolerant varieties. Yangtsipa is normally sufficiently tolerant to TLB.
• 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 management

Chemical control should no longer be necessary to control this disease as tolerant variety is available. Contact NPPC if there is a serious outbreak.

Version: NPPC 2017. Turcicum leaf blight V1.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 14 April 2017. Contact: nppcsemtokha@gmail.com

Image acknowledgements: NPPC.