Maize for Silage
Technical
Insight 339

NORTHERN LEAF BLIGHT

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CAUSAL ORGANISM:

Exserohilum turcicum (previously called Helminthosporium turcicum)

Grey-green, elliptical disease lesions begin to develop 1 to 2 weeks after infection (Figure 1). These lesions become pale gray to tan as they enlarge in size from 25 mm to 150 mm or longer (Figure 2). The distinct cigar-shaped lesions are generally unrestricted by leaf veins making Northern Leaf Blight (NLB) one of the easiest diseases to identify. Under moist conditions, the lesions produce dark gray spores, usually on the lower leaf surface, giving the lesions a "dirty" appearance. As many lesions enlarge and coalesce, entire leaves or leaf areas may be covered (Figure 3). With favorable weather conditions, new NLB lesions can produce spores in as little as one week.

Infections by NLB can occur at any growth stage during the season, but plants are more susceptible to infection after flowering. The earlier lesions develop, the more leaf area is reduced and the more damage results. Damage may include yield losses due to decreased photosynthesis, and harvest losses if secondary stalk rot infection and stalk lodging accompany the loss of leaf area. 

Figure 1: Early NLB lesions

Figure 2: Later NLB lesions and Figure 3 (Right): NLB lesions begin to encompass lower leaves

         

Epidemiology:

Northern corn leaf blight (NLB) is caused by the fungus Exserohilum turcicum, previously classified as Helminthosporium turcicum. It overwinters as mycelia and conidia in diseased maize leaves, husks and other plant parts. Spores are produced on this crop residue when environmental conditions become favourable in the spring and early summer. These spores are spread by rain splash and air currents to the leaves of new crop plants, where primary infections are produced.

Developing crop leaf tissue remaining wet for continuous periods exceeding 12 hours, together with temperatures in the range of 18 to 27C, favour spores being able to “take root” and establish an “infection site” on maize leaf tissue.

Secondary infection readily occurs from plant to plant, and even from field to field as spores are carried long distances by the wind. Infections generally begin on lower leaves first and then progress up the plant. However, in severe NLB outbreak years with high spore loads, infections may begin in the upper plant canopy. Heavy dews, frequent light showers, high humidity, and moderate temperatures favour the spread of the disease 

Figure 4: NLB disease cycle

In New Zealand, NLB infection may occur from early December onwards.

APPLY SOUND AGRONOMIC PRACTICES

Current farming practice normally offers crop plants the very best opportunity to grow and perform to a high standard of health and therefore productivity. The following agronomic principles should be applied to all maize crops in order to place the crop in  “optimum” conditions for the production of high yielding crops free of “stress”;

  • Seedbed preparation.
  • Soil drainage.
  • Nutrient balancing.
  • Uniform planting at the correct plant population.
  • Insect control.
  • Weed control.

CONTROL METHODS

1. Plant Resistance

Plant breeders have been able to identify specific genes that offer increased resistance when bred into plant lines. This transfer of resistance is able to be accomplished using standard plant breeding methods.

The Pioneer® brand maize hybrid product catalogue provides a list of plant characteristics and disease ratings for each hybrid, including NLB.

In conditions where NLB risk is high, growers should consider planting hybrids with at least moderate resistance ratings of 5 or higher for NLB.

NLB is not normally considered a problem for crops grown for silage as the main period of leaf tissue damage caused by the disease generally occurs after the timing of silage harvest but prior to grain harvest.

Outbreaks of this disease are normally associated with the northern areas of the North Island and in areas where maize crops are grown in the same ground year after year for grain harvest. The spores over-winter in the crop “stubble” and any stubble left on the soil surface offers the opportunity for infection to be transferred to the new season crop. Unfortunately there are the occasions where disease is wind blown and infects silage crops grown on “fresh ground”. 

2. Fungicide Application

Few fungicides are registered for the control of this disease in New Zealand. Unfortunately, by the time infection is noticed, the crop is normally too tall for ground spray equipment to be used for application. Aerial application is also expensive.

Application of chemical is normally required prior to tasselling.

  • Application proves effective by applying an early protectant shield of fungicide to all surfaces of the plant to avoid further infection (lesions) occurring. Repeat applications may be required if weather conditions favour continuing infection.
  • Early application allows time for the recommended withholding periods to expire prior to harvest of crops to be ensiled.

Fungicides do not kill the spores. They offer protection only to the unaffected portions of the leaf.

LATE PLANTED CROPS

Crops planted late in November though to early December are at considerably more risk to NLB infection and at an earlier stage of crop development. This increased risk results from the presence of high spore volumes from earlier planted crops which serve as a reservoir of spores. As a result it is important, in areas with a history of NLB incidence, that hybrids with high levels of resistance are selected for late season plantings.

Crops planted late to susceptible hybrids in seasons with favourable weather conditions, for NLB development, may benefit from the application of fungicides. It is important to use fungicides that are registered for the control of NLB in New Zealand and to follow carefully the conditions on the label. 

STUBBLE MANAGEMENT AND CROP INSPECTIONS

Early post harvest shredding and incorporation of plant stubble will go a long way to reducing the numbers of viable spores carried over from one season to the next.

Early detection of crop infection offers the opportunity to undertake a preventative fungicide programme when conditions favour the disease. Regular checking of crops in the December/January period will enable early detection of the disease.



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The information in this publication is general in nature only. Although the information in this publication is believed to be accurate, no liability (whether as a result of negligence or otherwise) is accepted for any loss of any kind that may arise from actions based on the contents of this publication.

© 2017, Genetic Technologies Limited. No part of this publication can be reproduced without prior written consent from Genetic Technologies Limited.

Revised: June 2015
Expires: June 2018