804 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



we have never seen, and probably it has not occurred in Britain again 

 during nearly half a century. 



Tubeuf, Dis. p. 518 ; Cooke, Hdbk. No. 1733 ; Sacc. Syll. iv. 2613. 



Cabbage Rot Mould. 

 Peronospora parasitica (Pers.), PI. VI. fig. 30. 



Sometimes found in company with the "white rust," and often 

 independently, on the leaves of many Cruciferous plants. 



We have already introduced this mould in the " Pests of the Flower 

 Garden," where it is far less troublesome and dangerous than here, and 

 to that account we have nothing to add, beyond the intimation that this 

 disease partakes essentially of the characters of the well-known Potato 

 disease, and that whatever remedies may have been applied with success 

 in one instance are likely to avail in the other. 



The only fungicide which has been recommended for use in the early 

 stages of this disease is diluted Bordeaux Mixture, but of course this will 

 be of no avail where the mould is well established and the mycelium has 

 penetrated deeply into the tissues of the host plant so that the resting 

 spores are in course of formation. In such case the only alternative is to 

 prevent the spread of disease by destroying all affected plants which may 

 contain resting spores. 



Known through the whole of Europe, the greater part of America, 

 and in Asiatic Siberia. 



For development of " rot moulds " see Introduction, p. 2. 



Sacc. Syll. vii. 830 ; Mass. B. F. 119 ; Mass. PI, Dis. p. 79 ; Smith, 

 Field Crops, 86 ; Gard. Chron. Nov. 17, 1883, figs. 109-111 ; Cooke, M. F. 

 f. 265 ; Cooke, Hdbk. No. 1778. 



Damping off. 

 Pythium De Baryanum (Hess.), PI. IX. fig. 99. 



This disease affects seedlings of cress, mustard, &c, when the plants 

 fall over and die off, as a result of the destruction of the fundamental 

 tissues by the attack of this parasite. The stem fails just above the sur- 

 face of the ground. 



The mycelium is branched, with the lateral branches thin and refiexed. 

 The conidia are globose, with thin walls, often terminating the lateral 

 branches (20 -30 /ti). The resting spores, or oospores, are also globose, 

 with a thick smooth outer coat (25-35 p) resulting from the conjugation 

 of a club-shaped cell or antheridium with the globose cell which after- 

 wards becomes the resting spore (fig. 99 a). 



In many features of their life- history these fungi, called Pythium, 

 resemble the rot moulds, and especially in the production of zoospores. 

 The resting spores, however, are produced externally, and not within the 

 tissues of the host plant. 



This disease only occurs in very damp situations, and should be pre- 

 vented by good drainage. 



Sacc. vii. 924 ; Ward, Dis. p. 88, figs. 5-9 ; Mass. PL Dis. 54, fig. 4 ; 

 Quart. Journ. Micr. Sri. xxiii. p. 487, t. 24, f. 1-10 ; Tubeuf, Dis. p. 116. 



