The rains this winter have left in their wake many winter forage fields devastated by various diseases and other problems. Probably the most prevalent problem has been that of waterlogging and nitrogen deficiency. Most all preplant fertilizer was washed out or denitrified (converted to nitrogen gas due to waterlogging) by the first set of rains early in the season. By the time the fields dried out enough to apply more nitrogen, there was no rain to take it in for a month and applied ammonia and urea fertilizers would likely have dissipated. Then in March, there was again too much rain, leaving the fields waterlogged again.
Most of the dried up lower leaves I have seen have been the result of waterlogging and nitrogen deficiency. On forage wheat, septoria tritici blotch, a leaf disease which is most favored by 2 or 3 days of continuously wet leaves and 60 to 70oF temperatures, is very severe. This disease causes grayish leaf lesions with tiny black specks in them. When conditions become dry and warm, this disease will not get any worse until the weather again turns mild and wet.
Another disease has been leaf rust. This disease forms many tiny rust-colored pustules on the leaves. The disease may become so severe that the plant is nearly defoliated. Leaf rust on wheat is favored by temperatures over 60oF and needs a film of moisture to develop. Dry weather retards its development. Planting forage wheat later in November is one of the best ways to avoid both these diseases, although this doesn't work every year.
One of the worst problems this season on forage wheat and some oat varieties has been physiologic leaf spot, which is affecting these crops throughout Northern California, Washington, Idaho and Oregon.
This disorder causes many yellow spots, necrosis, and irregularly-shaped lesions on the leaves. The leaf spots do not contain the black specks typical of septoria tritici blotch nor the pustules typical of leaf rust. Fields appear yellow from a distance, however, the stunting and poor growth is hard to separate from nitrogen deficiency and waterlogging. Plant pathologists have so far been unable to determine the cause of the disease.
None of these diseases should cause any toxicity problems with cattle, however, yields may be reduced substantially. On Dirkwin wheat severely affected by rust in my trials in a similar situation in 1993, harvesting at heading gave exactly the same yields as waiting another 8 days until the late flower stage.
November 5, 1999