300 Journal of Agricultural Research voi. xvi, no. h 



better would be the potatoes. The current reasons for this are many and 

 varied, but the principal one given is that there is less blight in the 

 soil. This may mean fewer fungus organisms in the soil, but that does 

 not seem to be the case. In several instances potatoes grown from good 

 stock on soil previously in alfalfa for nine years have been obsers'^ed as 

 badly diseased as the same seed on soil only one year in alfalfa. The 

 organism was present as abundantly as ever, and wherever the condition 

 of poor cultivation or heated soil was present, the disease was manifest. 

 The true value of alfalfa preceding potatoes lies in the fertilizer incre- 

 ment and mechanical improvement added to the soil, and not to any 

 dearth of Fusarium spp. 



The use of whole seed is suggested, not as a means of controlling the 

 blight, but of avoiding it. By the use of whole seed is meant not culls 

 and other small potatoes, but tubers in good condition, well selected, 

 and preferably of iK-ounce weight or greater. Whole seed has been 

 many times condemned as yielding quantities of unmarketable small 

 potatoes, and from the horticultural point of view this is a serious fault. 

 Under irrigation, however, the writer believes that whole seed can be 

 made to yield nearly as many marketable tubers as cut seed. The 

 increased stand resulting and the fact that no labor is required in 

 cutting would promise a return commensurate with the initial increased 

 cost of the seed. In one commercial field in 191 8 the yield from Avhole 

 seed was 100 per cent greater than that from cut seed of the same 

 variet3^ This field is shown in plate 41, B. The cut seed was planted 

 on the left and the whole seed on the right. The photograph was taken 

 at midseason. Sandsten {13) believes that whole seed is preferable to 

 cut seed in dry-land farming because it prevents seed-piece rot. 



SUMMARY 



The disease of potatoes in the field caused by Fusarium spp., princi- 

 pally F. oxysporum, whereby death of the plant or decay of any part of 

 it is brought about, is to be regarded as different phases of the same 

 disease. For that reason it is desirable to apply a generally applicable 

 name covering all stages. The term " Fusarium-blight " expresses this 

 adequately. 



Two methods of infection are recognized: Infection from the soil of 

 roots and root hairs, and infection of the seed piece, whereby the plant 

 becomes diseased. The latter method is regarded as the most serious 

 and responsive to environmental conditions in the Greeley district of 

 Colorado. 



Three methods of control are suggested, none of which have yet 

 proved wholly effective. First, selection for disease resistance, a method 

 shown to be effective only to a minor degree. Second, superior cultural 

 conditions for the potato plant, whereby it may always maintain a 

 degree of resistance to pathogenic organisms through activity and 



