Mildew on Wheat Seed - What and Why?

When we think of "mildews" in crop production we often associate this term with a group of plant diseases or pathogens such as the powdery mildews or downy mildews. However, there are many "mildew" fungi which are not parasitic on plants or do not cause disease but are happy to live on the surface of the plant feeding on plant excretions or organic matter. We refer to these as saprophytic or epiphytic fungi. Under certain conditions these harmless fungi can switch from their saprophytic ways to become problematic through direct penetration of plant tissues such as seed but overall they are more opportunists' rather then great pathogens!

Regardless of whether these fungi cause disease or not the main characteristic of these "mildews" is the superficial growth of the fungus (or fungi) on the plant surface, seed, head or other plant parts. This superficial growth can have an impact on wheat "grading" as we have seen this year. Mildew when used in the grading system refers to the greyish discolouration of the brush end (brush hairs) of the wheat seed and is caused by various fungi such as Cladosporium and Alternaria alternata.

These fungi thrive under wet, humid conditions and can be found on most any wheat plant at any time of the year (ubiquitous). Therefore, mildew damage can be found at some level each year in Ontario but are most noticeable in years when harvest is delayed. Under these conditions, we often see more mildew in harvested wheat samples. According to the Canadian Grain Commission website "a subjective estimation, based on overall sample appearance, is the only means of estimating moderate mildew damage."

Mildew is not associated with toxicological risks to humans and overall the impact on quality factors is minor as the Canadian Grain Commission website states "moderate mildew damage is apparently not a major quality factor". There are some flour colour, end product and export considerations however.

Unfortunately, there is very little that can be done at this time other then harvest fields as quickly as possible or if on-farm storage is available wait to market your wheat at a later date.

Figure 1. In this field of wheat, the grey of the mildew on the heads is clearly visible.
(Photo Courtesy of the Canadian Grain Commission)

Figure 2. Mildew damage on individual kernels of Wheat, Canada Western Red Spring.
(Photo Courtesy of the Canadian Grain Commission)