Primary symptoms
Many diseases which attack seeds and seedlings reduce plant density and plant vigor. Stalk rots can kill plants if they attack early in the cycle, and thus reduce plant density. Later in the cycle, stalk rots cause lodging. Leaf blights cause a reduction in photosynthesizing leaf area.
Confirm the problem by checking below.
Summary
Many diseases which attack seeds and seedlings reduce plant density and plant vigor. Since plant density is critical to good maize yields, these diseases need to be identified and controlled. In many cases, the impact of these diseases can be reduced by changing agronomic practices. For example, diseases which are unimportant when conditions are good at planting may become lethal if the seed is planted when soil temperatures are low «15°C).
Stalk rots can kill plants if they attack early in the cycle, and thus reduce plant density. Later in the cycle, stalk rots cause lodging.
Leaf blights cause a reduction in photosynthesizing leaf area. Their initial effects tend to be similar to defoliation, though later the production of toxins may become important. Leaf blights can resemble nutrient stress in the very early stages of the disease, but later in the cycle they can usually be identified using a disease guide.
| Causes of plant disease | Additional evidence required |
|---|---|
| Seeds were planted too deep, or in cool weather without fungicide. | Check planting depth and look at temperature records. |
| The variety is susceptible to important diseases in the area. | Ask the farmer what variety was planted. Ask if other varieties are less affected. |
| An alternate host for the disease is present in the area. | Look for other plants (or crop residues) which can serve as a co-host for the disease. |
Are diseases a problem?
Evidence: measurements.
Estimate plant density around flowering time. Is the density appropriate for the variety?
Evidence: calculations.
Ask the farmer how much seed was planted. Divide the number of plants by the number of seeds to get the percentage survival. If the seed was good (germination above 90%), you should expect about 80% survival to flowering. (Remember that soil insects and moisture stress can also reduce survival).
Evidence: observations.
-
Before the V4 stage: Look over the field for areas where emergence is poor. Dig up the seeds. Are they swollen with water but rotten or blackened? This is a sign of disease. The economic significance will be related primarily to the decrease in plant density.
-
Before the V8 stage: Look for plants which appear to be wilted, even though soil water seems adequate, or which are turning brown and dying. Pull up the plants and cut them open all along the stalk. Look for brownish spots or areas that appear to be water-soaked. These are signs of stalk rot. The economic significance will be related primarily to the decrease in plant density.
-
Near maturity: Make a count of stalk lodging. If lodging is significant, cut open some stalks and look for browning and shredding of the stalk tissues. The main economic significance will be associated with lodging, but if the disease attacked the plant early in the cycle, there may be a significant direct effect on yield as well. Is there significant leaf area above the ear lost to leaf blights? The area affected can be related to losses as in the case of defoliation, for a minimum estimate of damage.
Possible solutions
Note: All solutions require accurate identification of the disease. Try the pest key or consult a pathologist or a disease guide if you are uncertain.
-
Apply a fungicide as a seed treatment to control diseases of the seed and seedling, and use good quality seed.
-
Plant a variety which is resistant to the problem disease. Considerable genetic variation in resistance exists for most major maize diseases.
-
Change the planting date or rotation.