432 GROWTH OF PLANTS 



on lily troubles. The author describes and suggests methods for control of 

 two virus diseases (mosaic and yellow flat) ; five fungal diseases or groups of 

 diseases (Botrytis blight, bulb rots, rust, stump rot, and foot rot); four 

 physiological troubles (limber neck, frost injury, non-infectious chloroses, 

 and brown-tip of leaves) ; and one insect pest (bulb mite) . He also men- 

 tions several other minor diseases described in the hterature. 



Because it is the most destructive disease of lilies, except for species and 

 varieties that are tolerant, mosaic deserves special mention. When it 

 attacks lily plants outside it causes complete degeneration in a year or 

 two in some species. The disease is transmitted from diseased to healthy 

 plants by Aphis gossypii Glover and spreads rapidly under conditions 

 favorable for this insect. It can also be transmitted by intergraftiag 

 between diseased and healthy plants and with some difficulty by scratch- 

 ing juice from diseased plants into tissue of healthy plants. Once the disease 

 enters a plant, there is no cure for it since it is carried by the bulb. The 

 main sjonptoms are yellow mottling of younger leaves, showing three 

 types in different species, deformation and stunting of flowers and more 

 or less stunting of the whole plant. The main remedy is roguing out and 

 destruction of all plants showing the disease. This must be done as soon 

 as the disease appears on a plant in order to prevent spreading by the vector. 

 Also the foliage takes on a deeper color when the plant sets flowers or 

 when the temperature is high, thereby masking the mottling. Control of 

 the insect vector by sprays of course slows the spread of the disease. It 

 spreads faster in some species than in others, because the insect prefers 

 these as food. It is not carried by seeds; consequently seedlings are disease- 

 free until infected from an external source. 



Although of less economic importance than the mosaic, the yellow flat 

 or lily rosette disease of L. longifloruin is of interest. It is carried by the 

 same vector as mosaic but cannot be transmitted mechanically. It causes 

 general chlorosis of the foliage rather than mottling. It also causes marked 

 stunting of the whole plant and rolling or t\visting of the outer portions of 

 the leaves, reduces or prevents flower set, and distorts the flower and modi- 

 fies the shape and size of the bulb. Later work '^ indicates that this disease 

 is of considerable economic importance in Easter lilies and can be trans- 

 mitted by the vector to several species and varieties. The symptoms of the 

 disease vary with the species, which makes it difficult to diagnose in some 

 cases. 



Botrytis blight is a very destructive disease, sometimes under favorable 

 conditions ruining an entire crop in a few days. There is considerable 

 variation between species and varieties as to susceptibility to the disease. 

 It may attack the top of the young plant and stop further growth, and 

 even in some cases destroy the bulb. It may merely spot the leaves. The 

 spots may increase in size in more susceptible forms under favorable condi- 

 tions until the whole leaf is destroyed and plants are without live foliage. 

 When the disease attacks the plants in late stages, the flowers are spotted, 



