The first link on the AHS Daylily Dictionary page for leaf streak is to a Florida Dept. of Agriculture article on leaf streak of daylilies. Quote; "Leaf streak is worse in spring and fall in Florida, when daytime temperatures do not exceed 90° F", so I imagine that's the origin of the statement.
For daylily rust, the environmental effects were studied at the U of Georgia and published as Mueller, D. S. and J. W. Buck. 2003. Effects of Light, Temperature, and Leaf Wetness Duration on Daylily Rust. Plant Dis. 87:442-445. There's a link to the article from the "Literature List" page on my site.
I have been intending to add a summary of their findings to the FAQ page on my site but haven't got around to it yet, too busy at work and tired when I get home! Basically the optimum temperature for germination of daylily rust summer spores was determined to be 22-24 degrees C (72-75F), although some germination also occurred at lower rates between 7C (45F) and 34C (93F). No spores germinated at 4C (39F) or 36C (97F). Once germinated, the spores' ability to cause an infection at the temperatures tested was highest at 22 degrees C (72F). It was very low at 10C (50F) or less and 30C (86F) or higher. The study was done in C (and we use C here in Canada) so I've converted the above temps to F using an online converter thus there is some rounding of the F temperatures.
Once the plant was infected, the development of disease within 15 days
was highest between 22C (72F) and 30C (86F). At 10C (50F) the number of pustules was about half that at 22C (72F), and none were produced at 36C (97F).
To germinate, daylily rust summer spores need continuous wetness
on the leaves for a minimum of 5 to 6 hours. The longer the period of wetness the more disease may develop. Also, longer periods of wetness may be needed for infection at cool temperatures. Light intensity is another factor, high light intensity decreases spore
germination.
Thus the study suggests that infection is most likely to happen between evening and early morning when the leaves may be wet, the temperature optimal and the light low.
After infection has occurred the environment has less effect on the rust.
There's some more information on the effect of temperature and light from another study (U of Guelph) on this page on my site:
http://web.ncf.ca/ah748/latent...
As well as environmental effects on the fungus (very important in plant diseases) there is also the potential of an effect on the plant's resistance. Certain rust resistance genes in other plants are affected by temperature. I don't think anyone has looked into this for daylily rust.
