Wheat disease observations from plant pathologist
By D. Bruce Bosley
Ned Tisserat, Colorado State Extension plant pathologist, has written observations about current wheat diseases.
"We have been receiving a few wheat virus samples. The most common virus so far is the High Plains virus, transmitted by the wheat curl mite. This is somewhat surprising because we usually find more wheat streak. Symptoms of these viruses should become more apparent during warmer weather. Remember, we are still accepting samples for free testing. You might ask why it is important to test for specific viruses. There are a number of good reasons. One is that it helps us determine which viruses are most prevalent during the year. We can also learn which of the viruses impact yield the most and on which varieties.
I have had no reports of leaf rust or stripe rust in Colorado. There may be a little powdery mildew in some of the irrigated fields but that is about it. Therefore, I see no reason to apply fungicides now. It appears that we won't have to worry about diseases later in the season (at flag leaf) either, but things could change, especially with the series of recent storms. Leaf rust has been reported in south-central Kansas, but to my knowledge, the Texas panhandle and western Oklahoma still is fairly clean."
There are many fungicides registered for treatment of rusts in wheat. Most fungicides are timed to be applied when the flag leaf has fully emerged but before flowering. Let me point out that all Colorado wheat disease studies to date have shown no yield or quality advantages for fungicide treatments compared to doing nothing when stripe rust is absent in wheat fields. Ned Tisserat and I plan to continue these studies until we have determined the benefits of fungicide treatments when stripe rust problems hit Colorado wheat.
Wheat stripe rust infections can occur rapidly when conditions are right and when spores are blown in from Texas and Oklahoma wheat fields. Contact me if you want to receive alerts for this and other pests through e-mail.
Alfalfa weevil larvae are now feeding in area alfalfa fields. Weevils pose economic levels of injury in some alfalfa fields in Colorado each year. Many fields only experience weevil problems infrequently. For this reason, it is best to scout your alfalfa for this and other pests regularly--up until first cutting.
There are two approved alfalfa weevil-scouting methods: Count weevils per stem or sweep net method. To use the stem count method, carefully cut 30 to 50 stems at ground level, turn the stems over and shake vigorously into a clean 5-gallon bucket. Count weevil larvae in the bucket and repeat several more times for each field.
The alfalfa weevil treatment threshold is reached when one weevil can be found per stem on 8 inch alfalfa when expected hay prices are $105/ton. The threshold increases to two weevils per stem when alfalfa is 20 inches high up until bud stage at this same market price. The treatment threshold is lower when alfalfa hay prices are higher or lower.
Regular rechecks may be required if weevil populations are close to but lower than the recommended treatment threshold.
Using a sweep net can also provide a reliable weevil population estimate. Using a sweep net is usually much quicker than counting stems and weevils per stem.
Please contact me, Bruce Bosley, about these pests or other cropping systems topics at 970-522-3200, ext. 285 at Sterling or 970-542-3540 at Fort Morgan.