Vegetable and Field Crops 169 



so diminish the vigor of the plant that it assumes a sickly 

 appearance, ceases to develop, and fails to head. The 

 affected parts usually succumb to offensive secondary rots, 

 the causal organism is released, and the soil is thus infected. 



The proof of the parasitic origin of this disease was made 

 by Woronin in 1873, after three years of intensive study. 



In general the precautions against its spread are those 

 suggested under the general heading of soil diseases. All 

 diseased refuse is infective, as is also manure to which it 

 has had access. Fields which are diseased remain so for 

 several years; therefore rotation to nonsusceptible crops 

 should be followed. Seedlings should be examined care- 

 fully to guard against the use of plants from an infected 

 seed bed, and special care should be taken to avoid infecting 

 the seed bed with soil, manure, or refuse. Air-slaked stone 

 lime, 75 bushels per acre, applied each year some weeks be- 

 fore planting, has given beneficial results. In New York, 

 where 90 bushels of shell lime were applied per acre, an ex- 

 cellent crop was grown, while upon untreated soil only 60 

 heads were cut from 472 plants. The Hollander and the blue 

 or red varieties of cabbage are said to show some resistance. 



Root-knot (nematodes) . — Root-knot in some respects 

 resembles club-root, but with smaller swellings. It is com- 

 mon on crucifers. 



Black-leg -^•'^' -^^ (P/?.oma lingam (Tode) Desm.). — Great- 

 est injury is done to cabbage and cauliflower, less to other 

 crucifers. Dark, sunken areas occur on the stems near the 

 ground; gradually the stem, and later the plant, dies. Spots 

 may also appear on leaves or petioles. The presence of pyc- 

 nidia in the spots is the really distinctive character. The dis- 

 ease is important chiefly in the East and North, though it has 

 been reported from the South and extreme West, as well as 

 from Europe and Australia. Infection may be carried or main- 

 tained by diseased refuse or on the seed. Seeds should be dis- 

 infected (pp. 240, 449), and healthy soil used for the seed bed. 



Black-mold (Alternaria brassiar (Berk.) Sacc). — Upon" 

 the cabbage, cauliflower, and collard, particularly in the 



