Maize seed and seedling blights plus poor stand establishment due to herbicide damage

PROF BRADLEY FLETT, CROP PROTECTION (PATHOLOGY), ARC-GRAIN CROPS INSTITUTE AND ELBÉ HUGO, WEED SCIENTIST, ARC-GRAIN CROPS INSTITUTE

In recent seasons producers have reported many problems regarding stand establishment with maize, particularly maize fields planted early in the season. There are a number of factors that on their own or in interacting with other factors, result in poor stand establishment.

The major challenge for any plant pathologist is establishing which of these factors the primary cause is. Often these issues are brought to our attention by various persons who do provide a complete background to the situation, making this a bit of a guessing game.

Like with any successful detective work, the more complete the information made available to the plant pathologist or agronomist, the easier and quicker one gets to the root of the problem.

Weather conditions, particularly wet and cold conditions, during and after planting, play a major role in the development of these problems. Factors that may result in poor stand establishment include wet, saturated soils, cold soil temperatures (generally below 15°C), late frost or cold injury, wind and windblown sand injury, nutrient deficiencies, low soil pH (generally localised parts within a field), insect and nematode damage, herbicide injury, seed decay and seedling blights.

Many affected seeds do not germinate and decay in the soil; seeds that do germinate may die and decay prior to emergence from the soil and others may germinate and emerge, but die shortly after emergence.

After emergence, seedlings may be stunted; yellowing/reddening of leaves may take place as well as scorching of leaves and seedling length variation. The maize crop is susceptible to various stresses which include herbicide and seedling blights that injure the seed, seed roots, and/or the mesocotyl.

Early stresses impact on the establishment of the maize stand because seedlings depend on the kernel’s energy reserves until the root system is well established. These reserves are reliant on a healthy mesocotyl to transfer energy to the seedling.

Damage to the maize seed or mesocotyl prior to secondary root development, results in seedling stunting or death. For a healthy seedling to develop, it is essential that seed, seed roots and mesocotyl are healthy.

Seed decay and seedling blights

Many of these factors can be avoided with management practices and control of soil-borne insects and nematodes. However, seed decay and seedling blights may often be the cause of poor maize stand establishment and are often secondary to one or a number of the above-mentioned factors.

Seed and seedling diseases are generally caused by one of or a complex of soil-inhabiting fungi such as Pythium spp., Fusarium spp., Rhizoctonia spp., Stenocarpella spp. and Penicillium spp. These fungi may not affect plant stand if conditions are favourable for crop germination and emergence. Conditions that are not favourable give these fungi the opportunity to attack the seed and developing seedlings.

These diseases can be reduced by planting seed with fungicide seed dressings. These seed dressings are already on the seed purchased from the seed company. Planting seed into warm soils (above 15°C) promotes rapid germination, emergence and seedling vigour. Seedling diseases are often an indication of some other problem predisposing the seeds or seedlings to infection.

Role of herbicide injury in seedling diseases

Pre-emergence herbicides (before weeds and crops have emerged) are still predominantly applied with planting each season to control mostly grass weeds in maize.

Herbicide damage will usually occur when detrimental environmental conditions prevail, such as cold, wet weather resulting in poorly drained soils. Emergence of seedlings will tend to slow down in these conditions, causing seedlings (especially the coleoptile) to “grow” longer in the “herbicide zone”.

Seedlings that show herbicide damage will be severely stressed, predisposing them to fungi and/or bacteria, causing seedling diseases. It is, however, very difficult to distinguish between the primary cause of symptoms of these diseases.

Herbicide injury will usually appear where herbicides were applied incorrectly either due to erroneous dosage rates or equipment failure. Grass crops, such as maize and sorghum, will show more herbicide damage where graminicides are applied, while broadleaved crops (soybeans, sunflowers) will show more damage where broadleaved herbicides have been applied.

Pre-emergence herbicide injury is also soil related and it seems that damage will be more severe in sandy soils than in clay soils. Certain maize cultivars are also more sensitive to some graminicides than others and producers should confirm a cultivar’s tolerance to herbicides with the relevant seed companies.

It is still the best policy to keep to the correct dosage rates recommended on each label for each herbicide and to ensure that spray equipment is correctly calibrated to minimise herbicide injury and possible predisposing to seedling diseases.

Ensuring good stand establishment

• Ensure a fine, even seedbed to provide consistent planting depth and optimum conditions for germination and seedling growth.
• Avoid early planting in cold, wet soils. Seed fungicide treatments provide 10 to 14 days protection against fungi.
• Plant cultivars that are known to have good seedling vigour, first. Cultivars vary in susceptibility to herbicide damage, tolerance to cold soils, resistance to pathogens and other stress factors.
• Plant high quality seed from tested seed lots. Take note of seed lot numbers and where each is planted. In this way you will be able to revisit specific seed lots should one encounter problems.
• Plant fields with better drainage first.
• Ensure soil pH is adequate. Remember that soil samples are an average for the field, but there may be areas within the land that have lower pH values than the rest of the land. This is often a problem on sandy soils.
• Ensure effective herbicide applications. Incorrect applications are a major cause of poor stand establishment and predisposition to pathogens.

For more information, contact the authors at FlettB@arc.agric.za or HugoE@arc.agric.za.