35 



70 different species of plants. Curly top of heels has also been 

 transmitted to many plants, but is confined to the western part 

 of this country. Aster yellows occurs in all parts of the U. S. 

 Peach \ellows has been known for loo \ears, and resembles 

 both aster yellows and curly top of beets, but although it has 

 been much studied, little definite is known about it. Strawberry 

 yellows is apparently distinct, als(j witches' bnxMiis of potatoes. 

 The yellows disease of boneset {Eupatorium perfoliatum) is 

 apparently distinct from aster yellows. The curly top of beets 

 and aster yellows are transmitted by leaf hoppers: strawberry 

 yellows by an aphid. It is suspected that peach yellows is also 

 transmitted by an insect. A study of peach yellows is beset 

 with many difticulties. among which are the following: 



1. Although about 200 kinds of insects are known to visit it, 

 the peach tree has very few insects which are peculiar to it 

 alone. The green peach aphis, the black peach aphis and the 

 tarnished plant bug have been tested to see if they are carriers 

 of the disease, but with negative results. 



2. The length of incubation period of the disease makes the 

 problem difficult. It takes at least a year for symptoms to 

 appear. 



3. The symptoms are not very distinct in the early stages of 

 the disease. 



4. The apparent recovery of diseased trees. If diseased 

 trees are cut down and sprouts appear, these may not show the 

 symptoms of yellows for several years. 



5. There is difficulty in getting peach seedlings for experiment, 

 for the young seedlings often go into a type of growth which 

 simulates yellows. 



6. It is hard to keep diseased material on hand, for the trees 

 die either directly from the disease or from attacks of borers. 



7. On account of their large size, it is hard to work with the 

 trees experimentally. They cannot be put in cages like her- 

 baceous plants. The same is the case with the spike disease of 

 sandal. 



8. Experiments with cuttings are hindered by the fact that 

 these are dormant in the winter, the season when there is more 

 time for work of this sort. 



Dr. Harper suggested the testing of the plant juices chemicalK' 

 to see if in this w^ay there would be any indication as to whether 



