SWEET PEA DISEASES AND THEIR CONTROL 135 



may linger for a long time, or indeed sometimes throughout the 

 entire growing season. However, such infected plants remain 

 very dwarfed, and are practically useless for flower production. 



Root Knot or Nematode, Heterodera radicicola (Greef) Muller. 



The disease is characterized by swellings on the roots. These 

 are either small swellings formed singly, in pairs, or in strings, thus 

 giving the affected root a beaded condition, or the swellings may be 

 very large so as to be mistaken for root nodules. However, these 

 galls cannot be mistaken for the normal root nodules, because the 

 latter are lobed and are attached at one end, whereas the root gall 

 produces a swelling of the entire surface of the part affected. 

 Infected plants usually linger for a long time, but they can be 

 distinguished by a thin growth and yellow, sickly looking leaves 

 and stems. The disease is introduced with infected manure or 

 with compost. 



Mosaic. 



Of all the greenhouse diseases, none challenges so much the skill 

 and attention of the grower as this trouble. The great difficulty 

 with mosaic is that we do not yet know the cause, although every 

 indication points to its being a disease which is being induced by a 

 specific parasite. Serious as this disease is, it may be kept in 

 check, provided, of course, the grower recognizes certain funda- 

 mental facts. Mosaic is carried from diseased into healthy plants 

 by the bites of insects. Aphids, especially, are great carriers of 

 mosaic. Ordinarily winter-flowering sweet peas are not sown 

 directly in the benches in the house. The seeds, as a rule, are at 

 first sown in pots which are kept in a cold frame outdoors. The 

 seedlings, as they grow up, are open to the attacks of the green 

 aphid and several other insects which carry the germ from neigh- 

 boring older sweet pea plants. As the young seedlings are taken 

 into the house to be planted permanently on the benches the disease 

 too is introduced with them. Moreover, as is usually the case, 

 some of the aphids are also brought in. These soon begin to 

 reproduce and to establish themselves well in the greenhouse. In 



