Pomona College Journal of Economic Botany 



41 



Insects. — Prof. Rolfs attributed much of the red-spot to the work of the 

 red spider, and this indeed is a very important factor in the scattering the 

 spores on the leaves and fruit as well as a means of inoculating those por- 

 tions of the tree. Other insects as young black scale, plant lice, as well as 

 the silver-mite, prove efficient distributors of this disease. 



General Articles on Wither-tip. 



Of the disease in California the California Cultivator states : "This dis- 

 ease has been quite severe on citrus trees in some parts of the state. Last 

 November it occurred in a number of groves near DeLand, causing much 

 damage. It killed back twigs and limbs even in otherwise healthy and well- 

 kept groves. It was noticed, in some cases, that the damage done by wither- 

 tip became evident after several weeks of dry weather following a period 

 of moist weather. It was also noticed that in a neighborhood where groves 

 were close together, the disease would frequently be severe in one grove 

 while at the same time a neighboring grove would remain free from it. 

 Within the same grove also there were individual trees and individual limbs 

 of a tree that were attacked, while other trees of the grove and other limbs 

 of the diseased trees remained uninjured. The difference in the action of 

 the fungus toward different limbs of the same tree is probably due to a 

 difference in the vitality of the individual limbs. Any cause rendering a 

 branch somewhat weak, such as previous heavy load of fruit, would most 

 render that branch susceptible to an attack of the fungus. The same would 

 doubtless be true in regard to the susceptibility or immunity to the disease of 

 the different trees in a grove. Cases occur that can he accounted for by 

 considering trees as individuals, each slightly different from the other. One 

 tree apparently just as healthy as another is, for some cause, not yet under- 

 stood, less resistent to this particular disease, and is therefore attacked, while 

 the tree next it may remain uninjured." — Cal. Cul. XXXII, p. 708, June, 1909. 



The condition, as stated above, is no doubt a common example of the 

 attack of the wither-tip, but such was not the case in the orange orchards 

 at Santa Paula. The trees were in excellent condition, with plenty of fer- 

 tilizer, water, and cultivation. There had been no frosts, and no hot weather 

 to weaken the trees or branches. A rain fell about three weeks before which 

 started the spores germinating and an attack resulted which did not omit 

 even the healthiest-looking and most vigorous trees. It was a case where 

 every tree was touched without regard to its vitality. 



Prof. Cook of Pomona College writes: "During the last two or three 

 years a new disease has appeared in our citrus orchards. It is the old well- 

 known wither-tip of Florida. It attacks all kinds of citrus trees. It is 

 probable that it is affecting many orchards without being known or recognized 

 at all. 



The disease pits the fruit ; causes the leaves to fall, and the terminal twigs 

 to wither. In some orchards it has done serious harm and would have be- 

 come very much more serious had it not been discovered and the remedy 

 applied."— Cal. Cul. XXXIII, p. 406, Nov., 1909. 



