February, 1922. 



SCIENTIFIC AGRICULTURE. 



203 



but only those on surfaces cut or uncut. 

 Nevertheless, it is a sound practice as a 

 general rule with doubtful seed tubers. In 

 this sense it was recommended. 



Group 4. — Diseases caused by 

 Phycomycetes. 



TIhe Phycomycetes are the alga-like fun- 

 gi, as a rule possessing non-septate veget- 

 ative mycelium and developing both sexual 

 and asexual spores. In some cases the m}-- 

 eelium is profuse and in others it is re- 

 duced to a minimum. Zoospores- or motile 

 spores are produced in the cases under con- 

 sideration. 



(a) Black Wart or Potato Canker. 



This is one of the most serious and viru- 

 lent of potato diseases and is now known 

 to occur in England, Scotland, Ireland, 

 Scandinavia, Germany, France. Italy and 

 it has been reported from Africa. It is 

 common, in Newfoundland but is prac- 

 tically confined to Pennsylvania on the 

 continent. In Pennsylvania the report by 

 McCiibbin showed that in 1920 the disease 

 occurred in 781 gardens in 53 towns of 9 

 counties, so that the area involved covers 

 approximately 3,000 acres ailthough the 

 actual areas total about 100 acres. 



The disease is known under various des- 

 criptive names, such as Cauliflower disease, 

 Black Scab, and Wart disease, but Potato' 

 Canker or Potato Wart are more commonly 

 accepted. Schilberszky discovered the di- 

 sease in Hungary in 1896 and attributed 

 it to Chrysophlyctis endohiotica,. Percival 

 in 1910 made further studies and named 

 the causal organism SyncJiytrium endohio- 

 ticum. Its appearance on this side of the 

 Atlantic was first announced by Gussow 

 in 1919 from Newfoundland material. 



Symptoms. 



On above-ground parts there are rarely 

 any sj^mptoms but occasionally the fungus 

 may gain entrance to the haulm, lower 

 axillary buds or leaves. If this happens 

 the infected tissue is stimulated to exces- 

 sive growth giving rise to variously shaped 

 excrescences. It is on the tubers that the 

 symptoms are usually found and here the 

 buds or eves are affected. Diseased buds 



are stimulated to such liypertrophic growth 

 that they may be no longer recognisable. If 



Fig. 10 Potato Canker. 

 Susceptible variety (Cumberland Ideal) showing 

 effect on tubers. .Two cankers on above-ground 



parts can be see'n. 

 (From Supplement 18 to Jour. Bd. Agric. Eng. 

 1919). 



the eye is affected late in the season of its 

 growth the nodular excrescences may be 

 noticeable onh^ on careful examination. 

 But if the eyes are attacked while the tuber 

 is quite young the excessive growth of the 

 tissues will give rise to nodular irregular 

 masses entirely unlike a tuber. (See Fig. 

 10). The demand for food on the part of 

 the organism will be so great that any 

 stored in the developing tuber will be used 

 up and the tissues will collapse. In this 

 necrotic condition lies the greatest danger. 

 It is impossible to harvest such diseased 

 tubers without leaving behind fragments, 

 which are filled with sporangia, in the soil. 

 The resting sporangia thus left behind may 



