244 DISEASES OF CROP-PLANTS 



Nature of the Attack. 



The seat of the disease is the vascular system of the root- 

 stock. It does not appear that the vessels are actually plugged 

 as in many Fusarium diseases, rather it is their walls which are 

 disintegrated. The discoloration of the vascular bundles which 

 ensues is most marked in the rootstock and in the lower part of 

 the pseudo-stem, but frequently may be traced upwards into the 

 leaf-stalks and even into the midribs of the blades. In advanced 

 stages of the disease the fungus is accompanied by bacteria which 

 may cause a more or less general decay of tissue, especially in 

 the rootstock. 



Mode of Infection. 



The fungus can enter from the soil either by means of the 

 infection of roots or by way of the cut surfaces produced in the 

 removal of suckers. Attempts to infect parts above ground do 

 not lead to general infection of the plant. In diseased stools 

 the mycelium extends through the connecting vessels to the newly 

 formed suckers, and the severance and distribution of these as 

 planting material is one of the principal factors in the spread of 

 the disease. Mulching with infested trash has also played a 

 perceptible part. The Fusarium spores can be conveyed to a 

 distance by wind, by drainage water, and in earth adhering to 

 implements and to the feet of men and animals, and in this 

 way give rise to new centres of infection in the soil. Various 

 parasitic species of Fusarium are known to be capable of existing 

 for many years in the soil, and in respect of this disease Ashby 

 records a case where re-infection occurred three years after the 

 field had been thoroughly cleared and limed. 



Symptoms. 



The external syrnptoms are in general such as follow the 

 stoppage of conduction in the vessels affected by the fungus, 

 i.e., they are the characters of wilting. Typically the leaves, 

 usually but not invariably in order of age, turn yellow from the 

 margin or develop yellow spots and then dry up in succession. 

 The last leaf often stands erect and green for a time before it 

 falls. Eventually the whole column falls and rots. In young 

 and actively growing suckers the leaves may droop and break 

 down while still green. The symptoms vary in this respect with 

 the season and the age of the plants. A further, but not wholly 

 characteristic symptom is the splitting of the outer leaf-sheaths. 

 The bunch of fruit, where growth proceeds so far, may show 

 signs of arrested development and some or all of the fingers turn 

 black and fail to ripen. On badly infested land the stools are 

 often severely dwarfed, and most of the shoots succumb before 

 they reach the fruiting stage. The external symptoms cannot 

 in every case be depended on to distinguish the Panama disease 



