670 THE farmers' handbook. 



buried. Seedlings should be carefully examined, and if they show any signs 

 of Club Root should be burnt. As the soil may remain contaminated for 

 some time after a crop is attacked, a rotation of crops should be made. 

 What the rotation should be the farmer will determine for himself, provided 

 that cabbages or other Cruciferae be not grown more often than once in 

 three years. 



Downy Mildew. 



This disease probably occurs wherever cabbages are grown, but as a rule 

 does not cause any serious loss except in seed-beds. 



The cause of the disease is a fungus, Peronospora parasitica (Pers.) 

 De Bary, which may attack any cruciferous plants. It first appears as a 

 whitish downy mould in spots mostly on the under side of the leaves, but 

 may overrun the plant-stem and leaves. The parts of the leaf immediately 

 surrounding the diseased area appears yellow and later turns brown and 

 dries up. Frequently a mottled appearance results. The hyphae of the 

 fungus live entirely within the tissues, and send out branches into the 

 open air, and on these are borne numerous spores or conidia, which 

 are readily carried from one plant to another by a gentle breeze or 

 by insects. The spores germinate and send their hyphae into the leaves 

 through the breathing pores (stomata). The disease is carried over the 

 winter by means of another kind of spore (oospore), which is much more 

 resistant to unfavourable weather and soil conditions than the conidia, and 

 these will re-establish the disease in the spring. 



The chief control measures are concerned with the seedlings. Any plants 

 found diseased and all remains of dead plants should be destroyed. The 

 seedlings should not be grown too thickly nor kept too wet in the seed-bed. 

 When attacked they should be thinned out and sprayed about every ten days 

 with a weak Bordeaux mixture (6 — 4 — 50). 



This disease is sometimes associated with another disease that is common 

 on Cruciferae, known as " White Rust," as it appears like a white crust on 

 the plants, and is due to the fungus Albugo (Cystopus) Candida (see " White 

 Rust of Turnip," page 514). The same conditions and precuations apply to 

 both diseases. 



Black Leg. 



This disease occurs on cabbages and cauliflowers, especially when 

 young. A fungus Phoma oleracea (Sacc.) is responsible for the condition. 

 Infection frequently takes place on the stem at the surface of the ground, 

 just below the junction of the leaves, and also at the margins of the leaves. 

 The disease spreads downwards to the roots and around the stem, often 

 completely girdling it, giving it a black appearance — hence the name. 

 Often the fibrous roots and the lateral roots are killed. Before the plant 

 dies, a purplish tint is developed in the foliage, and persists until the plant 

 dies. Wilting of the plant is very characteristic, the leaves adhering to the 

 stem and drooping. On the leaves sunken spots occur, and later these dead 

 areas become covered with numerous minute black specks, which are the 

 pycnidia of the fungus enclosing numerous spores. It is harboured for the 

 most part in the soil on decayed stems and leaves of plants. Hence, in 

 preventing the disease, the need is shown, as is usually the case, of collect- 

 ing all refuse of present and past crops, and burning it. No leaves or parts 

 of diseased plants should be thrown on manure heaps, to harbour the 

 disease for the next crop. Seed-beds can be sprayed with weak Bordeaux 

 mixture (6 — 4 — 50), at the rate of 1 gallon to each 10 square feet of bed 

 space, and only clean seedlings should be planted out. 



