November, 1921 



sriENTlFIC ACKlfll.TlKI-; 



potato patches in orcU'r to reduce the pos- 

 sibility of infection by means of aphids. 

 It follows that the seed tubers used should 

 be healthy as far as it is possible to as 

 certain this fact. If diseased plants are 

 observed they should be rogued at once 

 and if aphids are seen on the diseased 

 plants wyien rogued a careful inspection 

 should be made of the plants immediately 

 surrounding. Any upon which aphids are 

 found should also be removed. Finally, 

 since aphids are the prime carriers of the 

 infective principle, it is necessary to con- 

 trol 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 fjae subject of mosaic, 

 mention should be made of a peculiar con- 

 dition found in severely diseased plants 

 by Gussow and also by the present writer. 

 In t,he leaves small, angular areas suddenly 

 become brownish and necrotic. In the 

 petioles brown streaks appear just below 

 the epidermis and later similar lesions 

 develop in the stem. Soon after the lesions 

 appear the petiole collapses and the leaf 

 Avithers, 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 t^iiis 

 is a type of severe mosaic, but the writer 

 has not observed similar conditions in 

 other than mosaic-diseased plants of the 

 CTreen Mountain variety. 



(c) Potato Leaf -roll. 



The disease known as "Leaf -roll" is. 

 like mosaic, comparatively new and in 

 many respects it resembles mosaic. It is 

 infectious, is transmitted by plant lice, it 

 is systemic, and is controlled in the same 

 way. 



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 diseases 

 and in cases of either excessive moisture 

 or continued drought. In true leaf -roll 

 the leaflets are distinctly rigid and not in 

 tjie least wilted. They are uprolled so that 

 the two margins tend to come together 

 forming a trough-like structure. Thev are 



ligliter green than iioniial k-aves and in 

 severe cases of tiie disease Ihey may be 

 yellowish, reddisli or ])urplish. The plants 

 are dwarfed, jiaving shorter petioles and 

 haulms than normal and the whole plant 

 has a typical aspect owing to the angle of 

 development of the petioles. Instead of 

 iK'ing rather spreading as in a healthy 

 plant the leaves and branc:hes tend to as- 

 sume an angle of approximately 45 degrees 

 fom the main stem. The stolons bearing 

 tubers are considerably shortened and the 

 tubers are smaller, fewer, and are cons- 

 picuously harder t^ian tubers from healthy 

 plants. This hardness of the tubers is 



Plate 5. — A leaf .showing- the dwarf, curled, 

 dark green s\mi)tom.s of mosaic. There are no 

 apparent light green patches. 



maintained even for cons'iderable periods 

 in storage. 



The rolling usually appears in the lower 

 leaves first but a late infection may affect 

 only the leaves last developed. 

 Effect on the Plant. 



As with mosaic the leaves of potatoes 

 suffering from leaf-roll do not function 

 properly. The cause of the disease lives 

 in the plant juices and in this case it es- 

 jiecially affects that part of the viiscular 



