I do not disagree with the observation that plants have temperature ranges that are optimum for their growth. However, the same can be said for pathogens - particularly those in the soil that cause root rots - Pythium sp, Phytopthera sp., Rhizoctonia sp., etc.. Many of these organisms have motile life stages and require free soil moisture (i.e. not bound by adsorption to soil particles, but present as capillary film between them). I have seen little discussion of the optimum temperature ranges for these organisms, but I suspect that it lies in the 65 to 80 F range - probably near the higher end of the range. However, their growth is likely reduced when the free soil moisture is reduced.
In the case of Trillium I have noticed that in nature the southern sessiles grow in well drained soil that can become quite dry in the summer. Sometimes the moisture in populations I observe is in the range of 10% of field capacity (saturation with capillary water present, but with no water dripping out by gravity) or less. These appear healthy with little or no root rot. In the garden in NJ the plants gradually decline with evidence of root and rhizome rots. They do this at soil temperatures lower than the summer temperatures typical of the Southern piedmont. This causes me to suspect that soil conditions that favor pathogens may be as important - or perhaps more important - then the soil temperature itself. John Gyer Clarksboro NJ USA
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