ISSUE 9 July 5, 2007
BLEACHED WHEAT SPIKES MAY NOT BE SCAB
In our travels this past week, we noted fields and plots in eastern North Dakota with numerous bleached-white spikes. These spikes looked very much like those caused by the wheat stem maggot. Nevertheless, they are easily distinguished from wheat stem maggot-damaged spikes as they are not easily pulled out of the plant at the first node, and there is no obvious insect feeding on the tissue at the base of the stem (see figures). These bleached spikes are somewhat similar in appearance to spikes that have been completely damaged by scab. Scab infected heads, however, are not so brilliantly white, but more pink to brownish, and often, if the whole head is infected, causes a discoloration in the stem right below the head, or kills only a few spikelets on a spike. Pink to salmon-colored spores and mycelium may be found on the margins of the glumes of spikelets infected with scab. These spikes do not appear to be the result of root rot as the damage was limited to the spike and not the entire plant and there was no evidence of root rot in the few plants we dug up. Root rot infected plants generally have a whitening or silver to bronze discoloration of the whole plant.
We believe that these spikes have been damaged by excessive heat shortly after emerging from the boot. The damaged spikes were most often found in the margins of the field where the lack of other plants increased evaporative demand, and in areas where there may have been excessive moisture earlier in the growing season. Though there was plenty of water in the soil profile to enable sufficient transpiration to protect the plant from the high temperatures encountered in July, the roots had not developed adequately to access this water. Water logging arrests root growth and when conditions become more favorable for root growth, the plant initiates new root growth rather than expanding the growth of previously damaged roots. The bleached spikes we observed were apparently damaged when transpiration demand exceeded the ability of the roots to provide moisture and temperatures were sufficient to dessicate the newly emerged spike.
Though the heat-affected spikes will not produce grain, the number of spikes in the fields we observed was relatively small and will probably not impact yield significantly. As hot dry days continue, plants with limited root growth in areas where the bleach spikes are prevalent may also experience water stress during grain filling and yield may be further impacted.
Heat damaged wheat spikes shortly after they emerged from the boot when roots were
not able to extract enough water from the soil to meet the evaporative demand of the plant in mid-June.
Wheat stem maggot; wheat head very white, and stem white down to first node; Pull head and node end chewed by maggot.
Root rot infected plants; stem as well as head have silver to bronze discoloration, and whole plant pulls easily from soil.
Scab infected heads
Extension Plant Pathologist