12 GROWTH OF PLANTS 



duce better flowers and continued production later in the fall by protection 

 against early frosts. The failure of the disease to appear in greenhouse 

 culture is probably due to the shyness of the insect carrier. 



With aster yellows out of the way, Dr. Kunkel tackled the century-old 

 mystery of peach yellows. Here he found again only one insect (Fig. 5), 

 a leaf hopper (Macropsis trimaculata Fitch), out of 15 commonly feeding 

 on the peach, capable of transmitting the disease. Dr. Albert Hartzell later 

 confirmed this conclusion on the specific carrier and found 47 other species 

 of insects and mites feeding on the peach incapable of transmitting the yel- 

 lows. Manns like\vise found ^ that M. trimaculata transmits the disease, and 

 by very limited tests concluded that the froghopper or spittle bug {Philaenus 

 leucophthalmus) is an even more effective vector. In 1942, Manns " seems 

 to throw some doubt on this conclusion. Hartzell states that peach yellows 

 has a necessary incubation period in the insect of 10 to 26 days with an aver- 

 age of 16 days, also that the nymphs rather than the adults are the main, 

 if not the sole, carriers of the disease. He implies that the low percentage 

 of transmission found by Kunkel and himself may be due to the fact that 

 individuals or strains of the species are unable to act as carriers, due perhaps 

 to the impermeability of their intestines to the causative agent and hence to 

 failure of the causative agent to enter the blood and finally the saliva of 

 the insect. The insect decidedly prefers various w^ild and even domestic 

 plums to the peach as food. Although plums take the disease and are in- 

 jured by it, the disease symptoms are largely masked in plums. The patchi- 

 ness of peach yellows on the peach both in time and location may be ex- 

 plained by the feeding preferences of the insect, the proximity of plum trees 

 to the peach orchards, and other ecological factors. 



Kunkel ^ later found that the causative agent can be killed in dormant 

 peach trees by heating the trees at various temperatures for the proper 

 length of time without seriously injuring the trees. Heating to 35° C 

 (95° F) * for 19 to 24 hours, to 50° C (122° F) for 10 minutes, or to 54° C 

 (129° F) for 1.5 minutes, the latter two in a water bath, destroys the infec- 

 tive agent without injury to the peach trees. The proposed methods for 

 control of yellows on peaches are: destruction of all diseased trees in the 

 orchard, removal of wild plums within a mile of the orchards, and contact 

 sprays to destroy the insect vector or vectors. 



Hildebrand, Berkeley, and Cation ^ mention about a score of other diseases 

 of the peach that resemble peach yellows in that they are transmitted artifi- 

 cially by blending of tissues, budding or grafting, and not by mechanical 

 transfer of juice. In nature the transmission must be by insects. Whether 

 the vectors are or are not specific for these diseases is not knouTi, for, in the 

 main, the vectors have not been determined. It will suffice here to mention 

 only a few of these diseases: rosette, little peach, red suture, phony disease, 



* The temperature first given for any reaction is that ased by the author of the article being 

 described, and may be either in Fahrenheit or in Centigrade, followed by the temperature 

 equivalent of the other scale in the nearest full unit. 



