74 PROTECTION OF PLANTS, 1921-22 



sized. If the grower prefers to use his own seed the seed plot should b& 

 isolated from other potato patches in order to reduce the possibility of infec- 

 tion by means of aphids. It follows that the seed tubers used should be 

 healthy as far as it is possible to ascertain this fact. If diseased plants are 

 observed they should be rogued at once and if aphids are seen on the diseased 

 plants when rogued a careful inspection should be made of the plants imme- 

 diately surrounding. Any plants upon which aphids are found should also be 

 removed. Finally, since aphids are the prime carriers of the infective principle 

 it is necessary to control plant lice. To accomplish this, add ^ of a pint of 

 "Black Leaf 40" to every 50 gallons of Bordeaux mixture and spray to cover 

 both the upper and under sides of the leaves. 



Before leaving the subject of mosaic, menticn should be mcde of a peculiar 

 condition found in severely diseased plants by Gusscw ?nd also by the present 

 writer. In the leaves small, angular areas suddenly become brownish and ne- 

 crotic. In the petioles brown streaks appear just helcw the epidermis and later 

 similar lesions develop in the stem. Soon after the lesions appear the petiole 

 collapses and the leaf withers, hanging by a mere thread. It then falls and as 

 the whole plant is rapidly affected a very characteristic appearance is given. 

 This is shown in Plate. 3 plant B. It cannot be stated for certain that this is a 

 type of severe mosaic, but the writer has not observed similar conditions in other 

 than mosaic diseased plants of the Green Mountain variety. 



(c) Potato Leaf roll. 



The disease known as "Leaf roll" is, like mosaic, comparatively new and in 

 many respect it resembles mosaic. It is infectious, is transmitted by plant lice,, 

 it is systemic, and is controlled in the same w^ay. 



Symptoms of the disease 



The name suggests the outstanding symptom but care must be exercised in 

 differentiating the rolling of the leaves in this disease from rolling in wilt dis- 

 eases and in cases of either excessive moisture or continued drought. In true 

 leaf roll the leaflets are distinctly rigid and not in the least wilted. They are 

 uprolled so that the two margins tend to come together forming a trough like 

 structure. They are lighter green than normal leaves and in severe cases of the 

 disease they may be yellowish, reddish or purplish. The plants are dwarfed 

 having shorter petioles and haulms than normal, and the whole plant has a typic- 

 al aspect owing to the angle of development of the petioles. Instead of being 

 rather spreading as in a healthy plant the leaves and branches tend to assume an 

 angle of approximately 45 degrees from the main stem. The stolons bearing 



