ROOT CROPS. 503 



Dry Rot and Wilt (Fusarium diseases). 



"These are two distinct fungus diseases of the potato, due to two closely 

 allied species of Fusarium. Dry Rol is generally found to be more prevalent 

 than Wilt. The potato (plant and tubers) is the host for about thirty 

 different forms of Fusarium, at least, twenty being distinct species. While 

 some are confined to the tubers, others occur in the stem parts. 



Dn/ Rut. — Avery common form of disease is a dry, whitisli, crumbling 

 condition of the tubers. An affected tuber, when cut through, often shows 

 a part* of the interior to have a putty-like appearanoe. If diseased tubers 

 are kept dry they mostly shrink up and become hard, whereas it' kept at all 

 moist or stored in a damp place, they soon become covered with an abundant 

 white, felt like growth <>f fungus threads. 



Investigations have proved that Dry Hot can be produced by many species 

 'it' Fusarium, some of which are true parasites, and can entirely destroy the 

 tubers, while others are saprophytes, which cannot by themselves destroy the 

 tubers, hut only assist in the destruction after the tubers are attacked by 

 other fungi or bacteria. The species causing Dry "Rot are not the same in 

 all countries. 



Wilt. — A species of Fusarium is typically found in the vascular (sap- 

 conducting) system of the potato plant, clogging up the conducting vessels 

 and so producing a wilting of the plant. It does not cause any Dry Rot of 

 the tuber, but sometimes makes its way into the stem end, where it may live 

 through the winter. 



When infected seedlings are planted, the result is a poor germination and 

 an uneven stand. The disease, however, does not usually attract attention 

 until the plants have attained the height of a foot or more. A wilting plant 

 shows a drooping of the lower leaves, which are the first to die. This is 

 followed by a wilting of the upper foliage, and by a premature dying of the 

 tops. Wilted plants are at first light green, then yellow, and finally dry up 

 and die. The presence of the fungus in the tubers is indicated, on cutting 

 across the stem end, by a dark ring a short distance below the surface. Wilt 

 of the growing plant and " stem-end ring " discolouration must be dis- 

 tinguished from Dry Rot of the tuber. This brown ring may be produced 

 by many of the other species of Fusarium that cause Dry Rot, and it is 

 often one of the symptoms of the presence in the vascular ring of many 

 species of Fusarium. Tt has been found that even when the brown ring is 

 visible, it may not be caused by a fungus, so that this condition cannot be 

 looked upon as absolute evidence of any fungus disease. The ring may not 

 always be present, as the fungi may gain entrance through wounds. The 

 disease gains entrance through the tender rootlets in the soil, gradually 

 working rip into the main roots, stolons, tubers, and some way into the stem. 

 In splitting open a diseased stem, the water-conducting vessels are found te 

 be slightly browned. 



Planting only sound tubers from a clean crop, and planting only on clean 

 land, are the most satisfactory control measures. Where there is any sus- 

 picion that the seed may be diseased, it would be advisable to cut all tubers, 

 and any showing a brown ring or any other suspicious marking should be 

 rejected, and so also should those showing any sign of surface rotting. As 

 the fungi can live over for some time in the soil, infected areas should have 

 a rotation of crops. Diseased plants and tubers should be burned. 



