78 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
of debris of vegetation in which the telial phase of the rust might 
have been distributed. This possibility would help to explain the 
uniformity in degrees of infection existing over entire field, as 
though irrigation water had been in some way related to the epi- 
demic, as well as the age of the crop plants. 
About a week after my visit, Dr. E. W. Olive examined the 
affected area and confirmed the above details from his observa- 
tions and interviews. In addition he learned from one or two of 
the more observing farmers that the aecidial sori had been seen 
scatteringly as early as late April of this year, and apparently the 
same trouble noticed, though doing no essential damage in other 
years. This would indicate that the aecidial stage (and by in- 
ference the other stages of the rust) has been present for some time 
as a parasite too insignificant as to damage to come to notice as a 
"disease" until the special conditions of weather and crop de- 
velopment made the outburst this season possible, as an epidemic. 
This would appear to be confirmed by the abundance of infections 
of the aecidial sori by the secondary parasite Tuberculina, which 
at that season would hardly find other hosts (rust) in abundance 
(sunflower, cocklebur and Bermuda-grass leaves with rust pustules 
appeared to be free from the Tuberculina). 
The sudden cessation of fresh infection with the passing of 
favorable weather conditions is in keeping with similar sensitive- 
ness among other rust species, and is one of the natural checks to 
the spread of such parasites. 
The above examples indicate the intimate relation that evi- 
dently exists between the healthy development of crop plants and 
the injuries caused by invasion of parasitic fungi producing disease 
conditions. The influence of climatological changes over con- 
siderable areas or during a number of days or weeks has been recog- 
nized for a long time, but the direct relation of small variations 
in limited areas has been less evident. In connection with diseases 
conveyed from season to season in planting seed (e. g., bean an- 
thracnose, cotton anthracnose), the saving of such seed from areas 
of lightest local rainfall, and during the most favorable period for 
seed-ripening becomes a practice of demonstrable value based on 
“crop hygiene.” 
