PERONOSPORA 107 



vessel full of water. The air of the Wardian case or of the 

 house should not be kept too damp.' 



Atkinson, Cornell Agric. Expt. Station, Bull. No. 94 

 (i895)- 



PERONOSPORA (Corda) 



Haustoria filiform, branched, conidiophores emerging 

 through the stomata of the host; conidia elliptical, apex 

 rounded, germination by the lateral protrusion of a germ-tube. 

 Oospores produced in the substance of the matrix. 



All the species are parasitic on plants, mostly on leaves. 

 Distinguished from allies by the filiform, branched haustoria, 

 and absence of a papilla at the apex of the conidium. 



A . / Fa II of oospore smooth. 



Onion mildew. This is due to the ravages of Peronospora 

 Schleideni (Ung), and is present in greater or less quantity 

 wherever onions are cultivated, and what is also important to 

 remember, it grows on various wild kinds of Allium. The 

 first indication of disease is the presence of small white 

 patches of mildew on the leaves, which within a couple of 

 days change to a greyish-lilac colour, and present a minutely 

 furry or velvety appearance. The side of the leaf opposite to 

 the patch of mould changes to a sickly yellow colour. When 

 once the mildew appears, if atmospheric conditions are 

 favourable, it spreads rapidly. A few days after the leaves 

 are first attacked they become dry, droop, and die, and a 

 ' neck ' or elongation of the plant between the bulb and the 

 leaves is a constant feature of the disease. The bulb is not 

 attacked, but if the disease appears somewhat early during 

 growth its size is much reduced. 



The conidial stage forms a dull lilac mould on the leaves ; 

 the conidiophores emerge in small numbers through the 

 stomata, and are erect and much branched near the top ; the 

 conidia are borne at the tips of curved branches, slightly 

 obovate, apex often more or less acute, pale, dingy lilac, 

 40-55x20-25 \i.. Oospores broadly elliptical or globose, 

 epispore thin, smooth, coloured. 



If onions are grown on land that has previously produced a 

 diseased crop, the probability is that the disease will appear 

 almost simultaneously over the greater part of the plot. On 



