DISEASES OF V\UIOUS CROPS 1711 



15) Having reaclfed the inner opening of a stomate the oospores 

 develop giving rise to two or three fine branches which emerge by the sto- 

 mate. As soon as these filaments are formed the}' can produce a terminal 

 spore, ovoid or lemon-sha,ped, which can also develop into a longer filament 

 with branches. Then the conidia are borne on the ends of the branches (ter- 

 minal spores) or in little swellings of the filament (lateral spores). 



16) The first conidia germinate as do the sporangia. Their contents 

 are differentiated into 8 distinct zoospores which are freed throiigh an opening 

 in the apex of the sporangium; they can begin their development as soon 

 as they are formed. The whole evolution, from the disintegration of the 

 chlorophjdl grains of the cell host to the time when the zoospores are li- 

 berated, is probably accomplished in one day. The fungus is propagated 

 after this stage by the zoospores. 



1231-The Disease of Potatoes known as "Leak", caused by Rhizopus nigri- 

 cans and Pythium de Baryanum. — Hawkins, l^. a., in journal of A<,ricultural 

 Research, Vol. VI, Xo 17, pp. 6^7-640, Fg. i, PI. XC. Washington, D. C, 1916. 



The tuber-rot of potatoes known as potato « leak « is a disease of 

 considerable importance in the delta region of San Joaquin River in Ca- 

 lifornia, where potatoes are cultivated on a very large scale. As a rule the 

 disease appears soon after harvesting, in the hot weather, in warehouses 

 and during transport and it often ruins whole shipments. Xo exact data 

 could be collected as to the losses but in 1915 for an area of 400 000 acres 

 the loss probably la^- between £. 10 410 and £. 31 250. 



The first sign of the disease is the appearance of a small brown disco- 

 loration around a wound of some kind, such as is easily made with a hoe. 

 In more advanced stages the entire surface of the potato is discoloured, 

 the tissues soften and shrivel; if tubers in this condition are crushed a 

 brownish waterv' liquid oozes out. 



Although Rhizopus nigricans Ehren. can produce a tviber-rot similar 

 to that occuring in leak, this fungus has not been isolated in field, experi- 

 ments from tubers affected with leak. In numerous cases Pyihium de 

 Baryanum Hesse has been isolated and must be regarded as the most common 

 specific cause of the disease; inoculation experiments have gi\'^en positive 

 results. The m^-celium is irregularly branched, with conidia borne either 

 terminally or intercalarty, they are spherical with an average diameter 

 of 22 fx. They germinate as soon as thej^ are placed in water at the ordinary 

 room temperature. The oogonia are spherical and vary from 15 to 25 jx 

 in diameter. The antheridia either occur on the same filament with the oo- 

 gonia or on an adjacent filament. The oospores are smooth, spherical 

 and thick-walled, with an average diameter of 16 [j,. 



Pythium de Baryanum is common throughout the delta region of 

 the San Joaquin River, and if infected soil is placedincont act with wound- 

 ed tubers they are readily infected with the disease. As a great many of 

 the injuries are ])robably inflicted with the hoe when the pgtatoes are dug, 

 great care in digging is recommended, and a careful sorting out of all the 

 wounded tubers. 



