326 



SCIENTIFIC AGRICULTURE 



June, 1922 



way to determine the efficacy and economy 

 of such a procedure. 



Storage Conditions 



Moist warm storage is suitable for the 

 rapid spread of dry rot (and other storage 

 rots) and it is therefore important to be 

 able to ventilate well and to maintain a 

 temperature about 35° F., or slightly lower 

 so long as the freezing point is not reached. 



Fusaria can live over the summer on refuse 

 in the corners and on the walls of bins, and 

 clean bins are essential to safety. 



Effect on plants 



If infected tubers are used for planting the 

 rot may continue so that no plant develops 

 or the shoots are weak and spindling. It is 

 entirely inadvisable to use even doubtful 

 seed. 



Control 



1. Avoid injury to tubers as much as 

 possible. 



2. Use cleaned and disinfected bins. 



3. Keep the storage places cool (35° F.), 

 well-ventilated, and dry. 



4. Avoid digging tubers for storage with 

 immature skins. 



5. Plant healthy seed tubers. 



(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 brownish 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. oxysponnn. 



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 arc 

 all gradations between these extremes. Slices 

 cut across the stem-end show that not only the 



vascular elements of the ring but als® the 

 secondary vascular strands in the cortex 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 vascular ring and cortex and 

 later to the ring only. Gradually the discol- 

 oration disappears 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 network of dis- 

 colored tissues is easily seen. Externally, 

 affected tubers appear to be sound but oc- 

 casionally the blackened bundles at the stem- 

 end may show through the skin. 



Effect on Plants 

 Such tubers used for seed purposes give 

 rise to spindling plants, the weakening of 

 the plant depending on the extent of vascular 



Fig. 14. — Photograph of half Petri dish show- 

 ing sclerotia developed in artificial culture on 

 on dextose agar 



necrosis. The basal buds are most affected 

 and the apical buds least. 



Control 



1. Plant only healthy seed. If in doubt 

 slice off the stem-end and discard tubers 

 showing strong discoloration. 



2. Sprout tubers before planting and 

 discard those with Meakly sprouts, or if 

 apical sprouts are strong while basal sprouts 

 are spindling cut off and discard the stem- 

 end. 



3. It is probable that dusting the cut sur- 

 face with finely ground sulphur would be 

 helpful. 



(h) Black dot disease 



Ducomet in France in 1908 reported the 

 occurrence for the first time of a disease 

 which he called "dartrosc" caused by Ver- 

 micularia various. McAlpine some two years 

 later found it in Victoria, Australia, and 



