Southern field crops (exclusive of forage plants) . ^cutting through the main root or stem;the layer just under the liark is l)lackene(.l,and throughout the stem the cut endsof the sto]iped-up water-carrying ductsappear as small dark dots (Fig. 181). 381. Spread and persistence of wilt. —Cotton wilt occurs chiefly in the sandyAND Healthy soils of theSouthern half of the cotton-CoTTON Stalks. |,p]^ ^his disease f^rst appears in small r^:t1:^in «P«*« i^ *^ fi^ll- It - extremely im-Rtfm attacked by portant for the farmer to recognize cotton^ *? wilt when it first ap]:)cars and while it is confine
Southern field crops (exclusive of forage plants) . ^cutting through the main root or stem;the layer just under the liark is l)lackene(.l,and throughout the stem the cut endsof the sto]iped-up water-carrying ductsappear as small dark dots (Fig. 181). 381. Spread and persistence of wilt. —Cotton wilt occurs chiefly in the sandyAND Healthy soils of theSouthern half of the cotton-CoTTON Stalks. |,p]^ ^his disease f^rst appears in small r^:t1:^in «P«*« i^ *^ fi^ll- It - extremely im-Rtfm attacked by portant for the farmer to recognize cotton^ *? wilt when it first ap]:)cars and while it is confined to these small spots, for these diseased areasenlarge rapidly every year when cotton is jilanted on thefield. In time the entire field becomes infected, and themajority of cotton plants of the ordinary varieties the field soon becomes useless for the cultivation ofthe common varieties of cotton. The germs of the disease Wve in the soil for four or moreyears, even when no cotton is Fig, 181. — SectionTHROUGH Wilted COTTOy DISEASES 413 382. Treatment of wilt by means of rotation of crops. —In spite of long life of the germ of cotton T\ilt, theonly effective treatment of the soil consists in starringthe germs. This is done to a considerable extent by keep-ing cotton out of the field for three years ; a longer banish-ment of cotton still more nearly gets rid of the the field may be used for corn, oats, grasses,the Iron variety of Qovrpeas, and certain other plants. It has been found that cotton wilt is most prevalenton soils which contain, not only the germs of the ?niltfungus, but also the minute worms that cause root-knot(see paragraph 385) on the roots of cotton and of numerousother plants. It is thought that the wounding of the rootsof cotton by these tiny nematode worms more readily per-mits the entrance of the germs of cotton ttilt. Hence,in a field where both troubles occur, no plants should begroT^Ti on which nematode worm
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