8 MASS. EXPERIMENT STATION BULLETIN 427 



temperature for these organisms was 79° and the maximum about 100°. White 

 (71) stated that the optimum temperature for growth was generally above 77°, 

 and Starkey (61) found that 80° was most favorable for infection. In Colorado 

 (60, 61) the greatest losses from root rot occur in October at minimum tempera- 

 tures somewhat above 50°. With temperatures maintained somewhat below 

 50° up to mid-December, the losses are reduced by more than one-half. 



The optimum temperature for the growth of Fiisariiim dianthi, the branch rot 

 organism, is about 80° F., the maximum about 95°. Van der BijI (67) found 

 that the optimum temperature was in a range of 77° - 86° and that no growth 

 occurred at 32° and 104°. Wickens (78) found that 84.2° was the optimum 

 temperature for growth in culture, strong growth occurred at 95°, and noiie at 

 50°. Bickerton (5) reported that the optimum, minimum, and maximum tem- 

 peratures arte approximately 80°, 43°, and 97° respectively. 



The optimum temperature for growth of Rhizoctonia solani, the stem rot 

 organism, is 86°-88° F. (53). High soil temperature and excessive soil moisture 

 are both extremeh' favorable. When both these conditions are present, infec- 

 tion and the progress of the disease are greater than in the presence of either 

 alone. The losses from stem rot are high in the months of July, August, and 

 September but relatively insignificant during the colder months and in the 

 warmer months of the spring season. With the advanced age and growth of 

 the plants the stem rot disease is almost totally absent. 



INFECTION 



The subject of infection of carnations by the various important wilt producing 

 pathogenes has been widely investigated and the important contributions on 

 the subject are reviewed here. 



Alternaria Spot, Blight, and Canker 



Severe infection not predisposed by injuries can occur at the leaf axils where 

 the fungus is favored by the retention of moisture (62). Corbett (21) observed 

 circular discolored areas with black centers on leaves and stems following inocula- 

 tion of carnation plants with a water suspension of spores of Alternaria, and the 

 inoculation of a freshly cut stem caused a dying back to the node and the wilting 

 of the adjacent branches. 



Injured plants are readily infected (62). Foliage injuries from drought, 

 chemicals applied to control pests, and infestations of red spider mite can con- 

 tribute to serious incidences of Alternaria blight in the field. The pointed tips 

 of the leaves sometimes die from drought conditions and then are invaded by 

 the fungus which subsequently progresses farther into the leaf blade. The 

 leaves of two potted carnation plants were injured by pricking them with a 

 needle; two other plants were not so injured. The plants were sprayed with 

 spores in water and incubated in a damp atmosphere. The needle-pricked plants 

 showed 117 distinct infections in contrast to 63 on the uninjured plants. 



The Alternaria blight disease can be readily produced artificially by spraying 

 carnation plants with a water suspension of spores of the fungus and incubating 

 the plants in a humid atmosphere. These single spore infections appear as 

 numerous small purple spots each surrounded by a halo (Fig. 4). Subsequently 

 areas in the leaves and flowers wither and turn brown and become covered with a 

 black mass of spores. Infection is readily obtained in this way on varieties 

 which show resistance under natural conditions in the field. Infection is also 

 readily obtained by inserting a piece of the fungus from culture in the axils of 

 the flowering branches and maintaining a moist swab of cotton about the point 

 of inoculation. 



