Feb. 16, 1920 Temperature Relations of Potato-Rot Fungi 52 1 



moderate optimum ; while the parasites of the southern zones (F. radicicola 

 and F. oxysporum) shift very decidedly toward both a higher optimum 

 and a higher maximum temperature.^ F. discolor var. sulphureum occu- 

 pies a somewhat intermediate position between these groups, which may 

 well explain its ubiquitous nature as a storage-rot organism. It has a 

 comparatively wide thermal range of growth with a moderate optimum 

 temperature. F. trichothecioides should be referred to the northern group 

 of the fungi, taking as a basis its thermal behavior in artificial cultures. 

 It differs, however, from the other two fungi of this group by its more 

 rapid growth at and somewhat below the optimum temperature. This 

 corresponds to the very active destruction caused by this fungus after it 

 becomes well established in the host tissues. The two Verticillia give a 

 particularly significant example of the correlation between the thermal 

 response of the fungi and their geogiaphical occurrence. The Northern 

 strain shows a better adaptation to lower temperatures and does not 

 grow above 30° C, while the southern strain shows a better adaptation to 

 higher temperatures and gives a fair growth at 30°. Their morphological 

 differences have not been sufficiently studied to warrant their separation 

 as two different species. It may be stated, however, that they are not 

 fully identical in pure cultures. Culture No. 426 has a tendency to pro- 

 duce numerous small sclerotia, which are practically absent in culture 

 No. 427. The difference in response between the two wilt-producing 

 fungi, F. oxysporum and V. albo-atrum No. 427, both with regard to 

 their lower and higher growth limits and their optimum points, was even 

 greater than with the two Verticillia. The results correlate with their 

 geographical distribution and explain why F. oxysporum, although present 

 in the soils of Maine, as well as on decaying tubers and in stem lesions of 

 the potato there, has, nevertheless, not been associated with potato-wilt 

 in that region. 



A correlation can be drawn also between the temperature relations and 

 the seasonal occurrence of Fusarium oxysporum and Verticillium albo- 

 atrum No. 427, as shown by the following experiments conducted at 

 Arlington Farm, Va. A certain number of tubers of the Irish Cobbler 

 variety were divided into three groups. One group was inoculated with 

 F. oxysporum, one with V. albo-atrum, and one was left uninoculated for 

 control. They were planted on the farm plots on April 27. A second 

 planting of similar groups was made on July 17. The first crop was dug 

 on August 26 and the second crop on October 21. Immediately after 

 digging the tubers were examined and wherever the stem-end discolora- 

 tion was present, isolations were made. The results of these isolations are 

 given in Table I. 



' The zones referred to should not be regarded as definitely limited geographical areas. The terms em- 

 ployed are relative only, and the areas overlap. They are determined not only by latitude but by elevation 

 and by soil climate. The fungi assigned to one zone may and usually do occur in the other. The assign- 

 ment has been determined largely by the relative prevalence and parasitic virulence of the organisms. 



