Weeds, Diseases and Pests

(and how to control them)

Weeds:

Good weed control is essential especially during April, May and June. This is because maize grows very slowly during these months and therefore cannot compete with the weeds. If weed control isn’t carried out at this stage, the yield may be reduced by up to 50%.

To control the weeds sprays are usually used, an example being Atrazine, which controls Orache, Black Nightshade, Knotgrass and Groundsel. It can be applied pre-sowing, pre-emergence and post-emergence at a rate of 2.5 – 5.01 litres/ha. However, the best time for application is post-sowing. It is effective for the first two years in which maize is grown on the same site. After this period, Orache and Black Nightshade become resistant. Since Atrazine is a residual chemical it will be banned in September 2005 and the use of contact sprays will be encouraged instead.

Another spray is called Stomp (against Orache, Black Nightshade, Knotgrass and nettles) but the use of this has also declined for the same reason. 3.75 – 5.01 l/ha of this chemical are required.

Atrazine and Stomp may also be mixed with other chemicals. Examples of mixes are Atrazine and Stomp (2.01 – 3.75 l/ha), and Atrazine and Lentagran (against Black Nightshade, Cleavers, Orache and Knotgrass).

To control docks, cleavers and voluntary potatoes, Starane 2 is commonly used (1.0 l/ha). Dow Shield is used for thistles and mayweed (0.5 – 1.0 l/ha) and Titus, which may also be mixed, is used to control grass.

Weed control if growing maize under plastic can be difficult. In this case, sprays are applied pre-sowing or during sowing.

Diseases:

Diseases reduce yield potential, alter normal maturity, reduce grain quality and cause stand ability problems. However, the growth stage or conditions of the plant determine if damage will limit yield potential.

1. Fusarium (F. culmorum, F. avenaceum and F. graminearum)
Attacks the stem base in August and September resulting in rotting or brittleness.
As a result of the stem damage, wilting, brackling (stem breakage at the point of cob attachment) and lodging are encouraged. Especially common along the coast and in warm conditions.
If the disease attacks early and the crop lodges, wilts or brackles harvesting difficulties may be experienced and there may also be a serious degree of yield loss.

2. Eyespot (Kabatiella Zea)
This disease usually strikes close to maturity.
It forces the farmer to harvest the maize earlier as a way of counteracting the effect of the rapid leaf loss.
First seen as small round translucent spots, which usually have a brown centre, which are surrounded by a purple/brown ring with a yellow ‘halo’.
Overwinters on crop debris and the spores which then develop will transfer to a crop via rain splashes. Therefore, ploughing is vital as eyespot attacks are encouraged if debris from the previous maize crop is left on the soil surface over the winter months.
Also transmitted by wind spores.
Significant yield and quality losses may be experienced.

3. Smut (Ustilago maydis)
Occurs when the weather is hot and dry.
This is a fungus that attacks the aerial parts of the plant and the stem base.
Evidence of the disease is the development ofare irregularly shaped galls. These have a white glistening skin and they produce a large mass of black spores. These spores are then blown onto other maize plants, which spreads the infection. Some spores may also remain in the soil.
The effect on yield and feeding quality is usually negligible.

Disease Control:

The incidence of plant disease in any given year is determined mostly by environmental and climatic conditions, although the following measures may also help to prevent disease:

Good, early and deep ploughing.
Crop rotation and ploughing combined can eradicate the existence of disease organisms as well as stalk rot. Crop rotation breaks up the life cycle of many disease-bearing organisms and should be considered if a build-up of a particular disease becomes evident.
By planting early, the maize plant can better compete with disease problems that occur later in the season.
Fertilise adequately to minimise additional stress on the crop, which will make it more susceptible to diseases.
Employ a good weed control system as some weeds may overwinter particular diseases.
Early harvesting is another preventative measure.

Pests:

The three main pests are leatherjackets, wireworms and corn borers, which attack maize if it is planted after grass. Leatherjackets and wireworms can be controlled using pesticides like Draza, Dursban and Gammalin, which must be sprayed onto the soil before the final tilling. Yaltox may be drilled with the seed. All seeds are treated with Mesurol, which helps with crow and frit fly control.

No fungicides are used, as they do not last long enough. Repeated spraying would be required when the maize crop is too high in order to use machinery. The use of machinery at that stage of growth would damage the stalks.

Leatherjacket

Wireworm

Corn Borer

Corn Borer Pupa in Stalk

Corn Borer Leaf Damage

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