October 



SCIENTIFIC AGRICULTURE 



59 



at the Agricultural College, at Charles- 

 wood, at Clandeboj^e, at Brandon by Mr. 

 I. L. Conners and at Morden by Miss Mar- 

 garet Newton. No extensive survey of the 

 province for this disease has been made. 



A good opportunity was provided dur- 

 ing 1920 and 1921 to study this disease in 

 a field of about 20 acres at the Agricultural 

 College, since the same field was planted 

 to sunflowers both years. This field was 

 near the Red River and had been i>rinci- 

 pally covered with brush. The field was 

 broken and sown to oats in 1919, then to 

 sunfloAvers in 1920 and 1921. These were 

 the first cultivated crops, except in one 

 small area in the field (where the disease 

 was worst) on which a farmhouse had once 

 stood. The soil in this area contained much 

 humus. 



The stem rot was first found in 1920 on 

 August 1st, It was found to be present 

 in 1921 on July 17th. The plants found 

 injured had all developed to 4 or 6 or 

 more feet in height. Temperature or other 

 factors seem to preclude attack upon seed- 

 lings or very young plants. The disease 

 usualh' attacks the plants at or near the 

 surface of the ground, and passes up and 

 dowij, sometimes reaching two or more feet 

 above ground under our conditions. Oc- 

 casionally, cases are found where the or- 

 ganism has attacked the plant one, two, or 

 more feet above ground, the stem being 

 healthy below. The affected area is at 

 first slightly sunken and black, but the 

 colour fades to a brown and finally to a 

 yellow. Mycelium of the fungus is usually 

 abundant, outside and within the stem ; 

 although under dry conditions, it may 

 not be evident on the exterior above the 

 ground. This mycelium becomes aggre- 

 gated into sclerotia which are white at 

 first, then black outside and white Avithin. 

 The mycelium spreads abundantly througli 

 or on the surface of the soil, and may be 

 seen conspicuously present under leaves, 

 or- where moist conditions prevail. The 

 plants wilt soon after attack by the fun- 

 gus (Fig. 1) and the stems are easily 

 broken at the injured area ; as the disease 

 progresses, the stem becomes shredded 

 from the ''retting" of the tissue, leaving 

 strands of fibro-vascular bundles. Event- 

 ually the sunflower plant is killed, and 

 dries out, thus checkinsr the growth of the 



fungus, and hard sclerotia of various 

 shapes and sizes are found abundantly 

 within the stem, especially in the area 

 l)reviously occupied by the pith, and clasp- 

 ing the stem on the outside. 



The disease occurred especially in patches 

 in the field, as noted above, in which often 

 practically every plant in a square rod or 

 more was killed. Here and there, through 

 the sunflower field, however, one oi- a few 

 plants in a place would be found affect- 

 ed. The composite weeds, Iva xanthifolia 

 and Canada Thistle (Cirsium arvense) 

 were found killed in the sunflower field, 

 but these weeds were less susceptible than 

 sunflowers. Sclerotia were present with- 

 in the stems, and isolations yielded the 

 same fungus as was obtained from sun- 

 flowers. Mr. I. L. Conners also found the 

 disease some distance away along the bank 

 of the Red River on wild sunflower, He- 

 lianthus tuherosus and on Sow Thistle 

 {Souchus arvense). Infection of the soil 

 in the field planted to sunflowers probably 

 came about either from having been pre- 

 sent on some vegetable once grown in a 

 garden patch in one part of the field or 

 from a natural infection of the soil by the 

 fungus liaving grown on some wild host 

 such as the wild sunflower. It may be 

 noted that in most cases where the disease 

 has been found in Manitoba, the sunflow- 

 ers were grown beside or planted after 

 vegetables. The same areas affected in 



Figure 2. — The development of mycelium 

 and sclerotia of the fun.srus on parsnip.s. 



