by Emile van den Berg – Agronomy Manager of Pioneer SA
Farmers find every year that maize plants fall over on some maize lands. This is caused by a series of stress factors which has an influence on stalk quality.
Drought, a lack of sunlight, insect and disease stress and low fertility are some of the most important stress factors which lowers the tempo of photosynthesis on the leaf surface area of the maize plant. When photosynthesis cannot provide in the development of the maize kernels, the plant withdraws root and stalk carbohydrates to the maize ear. This enables stalk rot organisms to invade the weakened and dying plant fibres.
Even positive growing conditions can result in stalk problems when it is succeeded by less favourable environmental conditions. Many additional factors, such as soil fertility, crop management and micro environmental effects, may aggravate the problem on certain lands.
There are a number of factors that may contribute to the falling over of maize, but it is mostly associated with a combination of weakened stalks as a result of a type of stress during grain fill, and stem rot pathogens which penetrate the weakened stalks. Weather conditions during drying-off may be a primary or contributing cause for stalks breaking. In many instances the already weakened stalks may break in high wind and heavy rain.
The leaf of the maize plant collects sunlight and carbon dioxide by means of photosynthesis to produce sugars (photosynthate). These sugars are transported to the actively growing fibre of the plant. During the early development of the plant the sugars move to the roots, where it is converted to structural carbohydrates and proteins in the developing root system.
Demand for carbohydrates during grain fill
Falling over problems with maize often appear because of stress during grain fill, which increases the amount of carbohydrates extracted from the stalk and roots. After successful pollination, the developing head has a major demand for carbohydrates. If the requirements of the developing kernels exceed the supply produced by the leaves, stalk and root reserves are extracted.
Environmental stress, which lowers the amount of photosynthate and energy produced by the plant, may even force plants to extract a bigger amount of stem carbohydrates, which is kept back by grain filling at the expense of stalk quality. High temperatures accelerates the development of plants, shortens the time towards maturity, and lowers the total amount of photosynthate product.
Factors lowering the functional leaf area, such as disease lesions, feeding insects or hail damage, also lowers the production of photosynthates. As carbohydrates, which is stored in the roots and stalks, is translocated to the head, these structures begin to regress and quickly lose their resistance and soil-transported pathogens.
Stalk rot often begins as root rot
Stalk rotting fungi is found in the soil in the root zone of maize plants and survive on nutrients discharged by the roots. This prevents metabolytes produced by the plant from penetrating the roots and stalks. Although this is not sufficiently virulent to attack healthy fibre, these opportunistic fungi lunge fast attacks on weakened and dying root material.
The above takes place when the plant translocates carbohydrates to the kernels during grain filling and the roots begin to decay. After the roots have been colonised, the infection spreads to the stalk.
As vascular tissue in the plant is blocked by the fungus mycelial, the water flow to the plant is restricted. Wilting and an early death of the plant finally follows. External discolouring of the lower part of the stalk is eventually seen clearly, depending on the regression of the internal stalk tissue. The stalk structure is weakened by this regression and putrefaction and the potential of the plant falling over is enhanced. Storm weather and high winds provide the necessary impetus for the weakened stalks to collapse.
Although pathogens play a key role in the development of stalk rot, it is especially the inability of the plant to provide enough photosynthase to the developing head that initiates this process.
Stress factors which may lead to weak stalks
- Drought stress
The drop of photosynthase resulting from drought stress has been well-documented. Water relations inside the plant, and CO² and oxygen exchange, is influenced directly. Furthermore – if leaf roll appears during drought, the effective leaf area for sunlight is lowered. Drought stress also cause a lowering of lignin deposit in the stalk wall, which weakens the wall appreciably.
- Low sun radiation
Photosynthesis is most effective in full sunlight. The tempo of photosynthesis increase in direct relation to the intensity of the sunlight. Photosynthesis may be lowered by as much as 50% on a cloudy day compared with a day of full sunshine. Extended cloudy conditions during grain filling often results in a serious withdrawal of stalk reserves.
- Soil fertility
It has been documented in research studies that soil fertility has a profound influence on the quality of the stalk. Most prominent are studies which indicate that a combination of high Nitrogen and low Potassium may influence the quality of the stalk detrimentally.
There is a relation between high Nitrogen and more kernels, which increases the demand for carbohydrates to the head. Higher Nitrogen also helps to extract these carbohydrates from the stalk and translocate them to the head by increasing the translocating tempo in the plant. The role of Potassium in the prevention of early plant death was established long ago. Potassium is active in the manufacturing of leaf and stalk matter and to regulate the movement of water within the plant. An increase in Potassium has a relation with increased photosynthesis. Increased Phosphorous also contributes to the lowering of lignin-binding in the stalk wall.
High-yielding hybrids may give very high yields, which come at the cost of late season stalk break. These hybrids translocate a higher percentage of carbohydrates from the stalk to the head during grain filling. This is noticed in a marked decrease of the wall thickness between flowering and maturity – a stalk shrinking process. This weakens the lower stalk joints, which may lead to falling-over, possibly after having been attacked by stalk rot diseases.