Cool, wet springs in many parts of the United States bring an unwelcome invader: anthracnose. This disease is known for causing defoliation of several tree species, sometimes multiple times in a single season. Because the disease typically does not kill or severely disfigure infected trees, it usually is tolerated rather than treated.

However, little effort has been given to breeding for resistant varieties despite the fact that this seems a problem well-suited for such a solution. Before breeding efforts can commence, it's necessary to understand the variability that exists among available species and varieties, from which to select desirable types for breeding. Researchers at the MortonArboretum recently sought to do so with ash trees, known to be susceptible to anthracnose.

Looking at 8 ash species, as well as multiple varieties of four species, the researchers found considerable variability in disease severity over the three-year study. Overall, white ash (Fraxinus americana) and blue ash (F. quadrangulata) were the most resistant species, with blue ash showing virtually no symtpoms. Chinese ash (F. chinensis) and green ash (F. pennsylvanica) were most susceptible. Similarly, varieties of the same species showed considerable variability in disease susceptibility.

Time of budbreak correlated with disease-severity tended to be somewhat worse with trees that leafed out earlier. The researchers admit that there is more than one potential explanation for this. However, one possible reason is that earlier leafing out exposed the foliage to more optimal disease conditions that exist earlier in the season. Alternatively, leafing out earlier may simply give the disease more time to infect (and reinfect) the foliage.

Regardless of the reason, these results show that considerable variability exists among ashes, meaning that breeders should have ample material from which to develop anthracnose-resistant varieties.

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