Growing Corn – Pests, Diseases and Disorders
What is the most important stage to control insects?
The critical period of insect pressure is from tasselling onwards; the major insect pest is the heliothis grub or corn earworm, Helicoverpa armigera . Eggs are laid around the flag leaves and on the silks when silks start to emerge. If insecticides are used, they must be directed to cover this area.
An Integrated Pest Management (IPM) system is essential to manage pests and diseases in sweet corn.
How do I adopt an IPM approach to pest management?
IPM requires the use of a number of components of pest management including pest monitoring by professional consultants, protecting beneficials, using biological insecticides, improving spray application equipment and making spray decisions based on pest and crop monitoring.
What causes stunting and deformed leaves in young plants?
The disorder, wallaby ear is the most likely cause of this problem. Wallaby ear is caused by maize leafhoppers that transmit a toxin into the plant. The higher the population of the leafhoppers the more toxin there is and the worse the stunting and distortion will be. Fifteen or more leafhoppers per plant on plants up to knee height will cause this disorder. Once the insect is bought under control new leaves that emerge will grow normally.
What causes striping on leaves?
Several things will cause leaf striping, for example a genetic breakdown, Johnson grass mosaic virus and maize stripe virus. Some nutrient deficiencies also cause striping; zinc and manganese deficiencies are the most common.
What causes powdery brown spots on the leaves?
A fungus known as rust. There are several rusts that affect sweet corn but the most usual one is common rust, Puccinia sorghi.
In moist weather brownish spots can also occur due to the partial germination of pollen.
What causes long grey narrow spots on the leaves?
The fungus turcicum leaf blight, Exserohilum turcicum. This disease is also known as northern leaf blight (NLB), and is referred to in USA literature as northern corn leaf blight (NCLB), often using its former name Helminthosporium turcicum. There are fungicides registered for use against it but they are protectants and will not have a curative effect. It is not generally economic to spray unless suitable weather conditions make a severe outbreak likely. If possible, plant resistant varieties. It is most prevalent in hot, wet weather and will not be a problem in dry conditions.
Do nematodes affect sweet corn?
Yes, particularly root lesion nematodes (Pratylenchus zeae). In north Queensland, lesion nematodes have been associated with poor growth and stunting in the field. Damage by lesion nematodes can often be diagnosed by the presence of small blackish lesions on the root surface.
Lesion nematodes are most active at soil temperatures around 30°C. Secondary infections of damaged roots by fungi and bacteria often make it difficult to estimate the losses caused by lesion nematodes.
Continuous cropping with sweet corn can contribute to a build-up of lesion nematodes in the soil. Sugar cane will also increase the numbers of Pratylenchus zeae resulting in damage if sweet corn is grown in the same soil immediately after sugar cane. Crop rotation with non-host crops is the best long term management strategy for nematode control in sweet corn.
