The Effect of Hail on Sunflower
The degree of hail tolerance in sunflower depends on the intensity of the hailstorm and the stage of growth: sunflower is least tolerant during the seedling and budding stages, and most tolerant after flowering.
Hail damage may be direct or indirect. Direct damage results from stand reduction, loss of recoverable heads because of severely bruised or broken stems, and head shatter at later stages. Indirect damage results from defoliation and disease infestation to injured plant tissue. Increased dead plant tissue resulting from a hail storm, especially on the back of a head, may increase the chance of Sclerotinia head rot infection.
Reduced yield as a result of defoliation depends on the amount of leaf loss and the stage at which it occurs. Stages R-1 (early bud) through R-6 (flowering is complete and ray flowers are wilting) appear to be the most sensitive to defoliation, since much of the plant’s energy at this time is directed to head development. At early and late stages of plant development, high levels of defoliation may not have a major impact on seed yield.
The degree of hail damage at different plant stages was illustrated a number of years ago by a sunflower date of planting study at Carrington, N.D., which turned into a hail damage study after one particular onerous late-summer storm.
Five sunflower hybrids sown at six planting dates between May 1 and June 20 were damaged by a hail storm on August 6. Stages of plant development at the time of the storm were from R-1 to R-7 (back of the head has started to turn a pale yellow color). Data were taken approximately one week after the storm. Average percent defoliation over all planting dates was similar at slightly over 25%. An average of 4.7 stalk and head stone bruises occurred per non-destroyed plant. The percent plants destroyed and the percent of the remaining plants with heads broken off or broken over but attached decreased with plant maturity (see table).
Other research conducted on simulated hail losses in sunflower has indicated that a one-to-one relationship does not exist between stand reduction and yield loss. A 50% stand reduction resulted in only a 28% yield reduction. Defoliation of sunflower by hail was reported to be most damaging during the bud stage. Defoliation of 80% at the bud stage resulted in yield reduction of 53%, whereas 80% defoliation at the 50% mature stage resulted in only a 12% yield loss. – North Dakota State University Extension Service
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