CHAPTER XXIX. 



MILDEW OF PARSNIPS. 



Peronospora nivea, Ung. 



THE descriptions we have already given of Peronospora 

 trifoliorum, D.By., P. exigua, W.Sm., P. Schleideniana, Ung., 

 and P. parasitica, Pers., more or less hold good with the 

 present species, which is often described imder the name 

 of P. umbelliferarum, Gasp. Peronospora nivea, Ung., affects 

 various umbelliferous plants, but is especially formidable 

 in its attacks on our garden parsnips. The fungus lives 

 within the tissues of the invaded plants, and attaches 

 itself to their constituent cells by minute suckers growing 

 from its somewhat torulose mycelium. Like other species 

 of Peronospora it causes putrescence in the plants it attacks, 

 and sets up decay in the leaves, stems, and roots. This 

 is very noticeable in the root of the garden parsnip. As 

 with the fungus of the potato disease, the mycelium often 

 descends the stem by the interior. In the parsnip the 

 large fleshy esculent tap roots become spotted and at length 

 putrid in a way similar with diseased potatoes. 



The parasite is very common on the Cow parsnip, or 

 hogweed, Heracleum, Spondylium, L. ; the wild Angelica, 

 Angelica sylvestris, L. ; and the Goutweed or Bishop weed, 

 jfigopodium Podagraria, L. 



The general appearance of this fungus is shown in Fig. 

 112, where a single fruiting-stem or conidiophore is en- 

 larged 400 diameters. The spores or conidia, as may 

 be seen, are supported on threads or spicules of extreme 

 tenuity. The point of junction between the parsnip leaf 

 and the fruiting branch of the Peronospora is shown at E. 



