June and early July are important months in terms of field crop disease development. For diseases such as sclerotinia stem rot of canola and ergot of small grain cereals, suitable pathogen development must occur to produce spores before host infection can take place. The pathogens associated with both diseases overwinter as resting bodies known as sclerotia, which are compact masses of hyphae (Figure 1). Germination of stem rot sclerotia to produce the golf-tee shaped apothecia, aka carpogenic germination, requires suitable moisture conditions and moderate temperatures:
- Typically, sclerotial germination takes up to three weeks with the occurrence of temperatures that range from 10 to 20oC and suitable moisture conditions
- Soil moisture levels at or close to field capacity at the soil surface are needed for germination of sclerotia, although some germination can occur when levels are somewhat less that field capacity
- Field capacity varies depending on soil texture but is generally around 20% soil moisture (volumetric water content [VMC], aka percent saturated soil) for sandy soils, while for heavier textured soils this can range up to 40 % soil moisture
PCDMN weekly soil moisture maps
The PCDMN has been working with Agriculture and Agri-Food Canada (AAFC) to obtain maps showing surface soil moisture based on satellite observations. These soil moisture values are in VMC and usually range from 5-45% by volume and the relationship between this value and the plant available water are affected by soil texture. For example, a dry sandy soil may have a volumetric moisture content as low as 5% when dry, whereas the lower bounds for a clay textured soil may be closer to 20% VMC. Soils with a VMC of 40-60% would be wet/saturated, although the level of saturation would vary based on soil texture, where a sandy soil may be saturated at 30% compared to 40-60% for heavier textured or organic soils.
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