Guide to diagnosis of chlorotic and poor vigor wheat in fall
If wheat does not emerge, the problem is usually due to dry soils, a hard crust on the soil surface, or seed with poor germination. If the wheat emerges, but is chlorotic or has poor seedling vigor, several factors may be the cause. Often, a combination of factors is involved.
The most common causes of yellowish, chlorotic wheat in the fall are:
* Poor root growth. This may be due to soils that are too dry, soils compacted by a heavy rain, seedlings buried by soil moving into the seed furrow, or poor seedbed conditions at planting time. If the plants have been emerged for several weeks or more, can be pulled up easily, and have only a couple primary roots visible, then the root systems are not extensive enough to provide enough nutrients. This will often cause yellowing.
* Nitrogen deficiency. In the seedling stage, nitrogen deficiency causes the plants to look pale or slightly chlorotic. Nitrogen deficiency also results in reduced tillering, top growth, and root growth in the fall. The primary causes of nitrogen deficiency in the fall are poor root growth, insufficient fertilizer rates, leaching from heavy rains, and the presence of heavy amounts of crop residue, which immobilize nitrogen.
* Leaf rust or tan spot. If leaf rust infects young seedlings in the fall, the plants may turn yellowish. Fall infections of leaf rust are not common in Kansas, but can occur. Producers will be able to see pustules on the leaves. Tan spot can also cause wheat to turn yellow in the fall. Viral diseases, such as soil-borne mosaic, wheat streak mosaic, and barley yellow dwarf, can infect wheat in the fall, but yellowing symptoms do not normally show up until early spring. It rarely, if ever, pays to treat fields in the fall for leaf rust or tan spot, even if those diseases do cause yellowing. Cold temperatures in the winter normally cure this problem.
* Cold temperatures. This has not been a factor yet this year, but if recently planted wheat has not yet emerged, it may not come up until the weather is cold. When temperatures are quite cold at the time wheat emerges, it can result in yellow banding on the leaves. If this is the cause of the yellowing, symptoms should eventually fade away.
* Mites. (See items in next section.)
Other poor vigor problems
Where wheat stands are thin and lack vigor but are not yellowish and chlorotic, the most common causes are drought stress and aluminum toxicity. Other possible causes include severe phosphorus (P) deficiency, insect or mite feeding injury, root rot and seedling diseases, and herbicide carryover problems.