310 



SCIENTIFIC AGRICULTURE. 



May. 1922 



Diseases of the Potato 



by B. T. DICKSON, 

 Professor of Botany, Macdonald College. 



(Continued) 



Group 7. Diseases caused by Fungi 

 Imperfecti (Adelomycetes or Deuteromy- 

 cetes). 



(a) Early Blight. 



This disease is common in years when 

 Late Blight does not occur and while it 

 does not attack the tubers the yield is 

 reduced because of leaf injury. The 

 amount of injury depends upon the sea- 

 son and it is greatest when hot spells al- 

 ternate with moist weather. In such cases 

 losses in yield may reach asi high as 25 

 percent, unless spraying >s consistently 

 practised. The disease is caused by a 

 fungus known as Alternaria solani (E. & 

 M.) J. and G. 



Symptoms. 



During the season the leaves are the 

 only parts attacked. The first sign of the 

 disease is a small yellowish spot with a 

 slightly watersoaked margin. The spot 

 enlarges until it is from 1-8 to 1-4 of an 

 inch in diameter and variations in the 

 growth rate of the fungus give rise to 

 concentric markings in the spots. The 

 whole becomes dried out and brownish- 

 black and it frequently happens that the 

 central part of the lesion falls or.t giving 

 rise to a modified "shot-hole" effect. 

 When the disease is severe owing to suit- 

 able infection conditions several spots may 

 coalesce. Under these circumstances the 

 foliage is considerabh' reduced and at 

 times very few mature leaflets will have 

 any chlorophyll bearing tissue with which 

 to function in manufacturing food for 

 plant growth and tuber development. Per- 

 sistence of tliese conditions will involve 

 the premature death of the vines. 



The stems are not affected until late in 

 the season and it is in debris from vines 

 and leaves that the fungus overwinters. 

 Life History. 



Tiie fungus may overwinter as mycelium 

 in old lesions in plant debris or as spores 

 but there is no known perfect stage. 

 Under moist conditions spores, which have 

 overwintered or have developed from over- 



wintered mycelium, infect leaves causing 

 the characteristic spots. On these dead 

 spots the fungus produces many conidio- 

 phores bearing rac(iuet-shaped, multicel- 

 lular spores. These, when washed or 

 blown to other leaflets, germinate rapidly 

 if moisture be present and so the disease 

 is spread throughout the season. 

 Other Hosts. 



Alternaria solani may also attack the 

 tomato. 



Control. 



The only method of control is consis- 

 tent spraying with Bordeaux as for Late 

 Blight. 



(b) Wilt or Fusariose. 



Luckily this disease, caused by Fusar- 

 ium oxysporum Schlect, is not of serious 

 economic importance here since the causal 

 organism is best suited to sandy loam 

 and a hot growing season. It is common 

 in Pennsylvania, Ohio, Michigan, Indiana, 

 South Dakota, etc., and it is sometimes 

 severe in south-eastern New York. The 

 fungus is to be found in our sandy-loam 

 soils and potatoes are attacked but wilting 

 to any marked extent does not often occur. 

 Symptoms. 



In early stages of the disease the plant 

 shows rolled, slightly yellowed foliage. 

 The rolling can be differentiated from 

 that in "Leaf roll" because the leaves are 

 not stiff and inclined to brittleness as in 

 the latter disease. If the attack is slight 

 rolling and yellowing of the leaves com- 

 bined with a dwarfing of the plant may 

 be the only symtoms until late in the sea- 

 son when such plants die down premature- 

 ly. If the invading organism develops 

 more rapidly the lowest leaves yelloAv and 

 fall and there is a tendency to "rosette- 

 top" quite similar to that in the Rhizoc- 

 tonia disease. Given optimum conditions 

 for the fungus and the vines lose their 

 lower leaves and wilt. In very severe 

 cases a lesion may be ol)vious just below 

 ground level but as a rule it is only on 

 cutting through the stem that one can see 

 the browned condition of the vascular 



