122 PHYCOMYCETES. 



same author also quotes the disease on Solarium caripense 

 at Quito, and on Petunia hyhrida at Upsala. 



The potato disease is above all an associate of moist 

 weather. In such circumstances, the conidia are produced 

 very rapidly and the zoospores readily distribute themselves in 

 the moist soil. There is thus greater risk to the potato crops 

 on wet soils. 



For wintering, potatoes as healthy as possible should be chosen. 

 This is particularly the case if the tubers are required as seed r 

 for the fungus-mycelium spreads from the tuber into the shoot. 

 Whole tubers are less liable to infection than those cut or 

 broken. Some varieties {e.g. thick-.skinned) are less easily 

 infected than others ; such should be selected and bred. 



As a preventive measure the leaves may be sprayed with 

 Bordeaux mixture, or with a copper carbonate mixture.^ By 

 these means conidia and zoospores which alight on the plants 

 are killed and their germination prevented. The leaves them- 

 selves remain uninjured if the copper compound be used dilute 

 enough. These compounds may even be beneficial to the growth 

 of the host-plant, as was found by Eumm^ for the vine, and 

 Frank and Kriiger^ for the potato. 



Frank and Kriiger found on using a two per cent, copper 

 sulphate and lime mixture, in which the copper is known to be 

 the potent constituent, that the potato leaves were stronger, their 

 chlorophyll-contents greater, their power of assimilation and 

 transpiration was increased, the life of the leaf was lengthened, 

 and the yield and starch-contents of the tubers were increased. 

 They regard the effect of the copper on the leaf as the result of 

 a chemotaxic stimulus. 



Jensen recommends disinfection of seed-potatoes by heating 

 at 40° C. for four hours. 



Ph. phaseoli, Thaxter, lives in young bean-pods and causes 

 them to shrivel up. The fungus is as yet incompletely known, 

 having only been observed in America where Thaxter * reports 

 great destruction of Lima bean {Phaseolus hmatus) near New 

 Haven. 



^ See also § 12. Detailed experiments of this kind are frequently described 

 in the magazines relating to agriculture. (Edit.) 

 ^Ber. d. deutsch. botan. Qes., 1895, p. 189. 

 ' Ber. d. deutsch. botan. Ges., 1894, p. 8. 

 ■■ Thaxter, Botanical Gazette, 1889. 



