98 



PROTECTION OF PLANTS, 1921-22 



(f) Net-necrosis 



The term "net-necrosis" was first used by Orton in 1914 to describe a 

 discoloration of the vascular tissues of the tubers. The color varies from brown- 

 ish to black and it may appear in the stem-end only or extend throughout the 

 tuber. There are now three accepted types of net-necrosis, viz: — Frost net- 

 necrosis, Fusarium net-necrosis and Leafroll net necrosis. It is also suggested 

 that a net-necrosis may be caused by adverse soil or growing conditions. 



In any of these cases it is inadvisable to use such tubers for seed purposes 

 since they will develop spindling plants. 



The net-necrosis under consideration is that which is the result of 

 Fusarium infection, usually F. oxysporum. 



Symptoms 



The discoloration of the vascular tissues varies from a slight browning at the 

 extreme stem-end to an intense darkening two-thirds or more through the tuber. 

 In very severe cases the discoloration may extend to the bud-end but usually 

 the distal half of the tuber is free from any symptoms. There are all gradations 

 between these extremes. Slices cut across the stem-end show that not only the 

 vascular elements of the ring but also the secondary vascular strands in the cor- 

 tex and medulla are affected. 



On cutting successive slices it is seen that the discoloration becomes less 

 intense as the bud-end is approached and that it is more confined to the vascu- 

 lar ring and cortex and later to the ring only. Gradually the discoloration dis- 

 appears and slight water-soaking in the vascular ring is the last symptom to- 

 wards the bud-end. By paring the tuber down to the vascular ring the net 

 work of discolored tissues is easily seen, Externally, affected tubers appear to 

 be sound, but occasionally, the blackened bundles at the stem-end may show 

 thro igh the skin. 



Effect on Plants 



** '.^'*iic^i 







Such tubers used for seed pur- 

 poses give rise to spindling plants, 

 the weakening of the plant de- 

 pending on the extent of vascular 

 necrosis. The basal buds are 

 most affected and the apical buds 

 least. 



Control 



Fig. 12 — Photograph of half Petri dish showing scle- 

 rotia developed in artificial culture on dextrose agar. 1. Plant only healthy seed. If 



in doubt slice off the stem-end and discard tubers showing strong discoloration. 



