Arizona republican., 1921-04-18
By J, G. BROWN
(Plant Pathologist, Arizona Agricu
 tural E,xner-inent Statinnl

ldIL <AIlZoild
 tural Experiment Station)
 During the past season the cotton growers of Arizona suffered consid- erable loss from two diseases which were prevalent in a large number of fields, namely, root rot and black arm Root rot, like the poor, we always have with us, but black arm probably can be completely controlled and at a comparatively small ex pense. It is the purpose of this article to discuss the latter disease and to set forth the methods of pre- Venton' and control
 Disease caused by the black arm organism appears in the field in sev- eral forms. It may show in the form of leSionS, or more commonly as ] blackened areas, on the stems of plants of various ages. It r1ay kill l the leaves and tender stems of seed l lings, turning both black. Sometimes 1 it appears to attack only the leaves. forming angular, water soaked areas
 between the veins, which la-er turn | black and give the name angular 1 leaf spot" to the disorder. Again, the
 disease affects the bolls, causing
 them to become rusty colored, bring
 ing on premature opening, and often
 staining the fiber light brown, thus
 reducing it's market value. All of ! these manifestations are due to the
 same organism, Bacterium nalvacea-
 rum. Invariably the first symptom is the same whether seedlings, ma ture leaves, stems, or bolls are at
 tackled, namely, the appearance of very small water soaked area which usually at first escape the eye of the
 farmer.
 It is not necessary that the cotton plant be killed to lower the yield oi
 the crop. Leaves infected with the
 bacterium of black-arm eventually
 become less efficient and since they are the organs that manufacture the material from which the fiber is pro-
 duced, their extensive infection
 means a less productive plant. Many
 of the leaves may be killed outright } and fall off, making the loss greater The grower seldom thinks what the
 loss of a few leaves per plant may
 amount to when the whole field is considered. Of course very leafs plant may lose some leaves withou damage, even to advantage in some cases, but the disease doesn't stol with the killing of a few leaves as ; . rule.
 The loss due to inefficient ant , fallen leaves plus that caused b1
 plants that are killed or so weakened  added to by the rotting of bolls anC the staining of fiber, may mean thu , difference between a profitable crol and a balance on the wrong side C . the ledger. In the South it is no , unusual for plants to lose as high a
 50 per cent of their leaves by tn ! first of August where fields ar badly attacked, and the number O l infected bolls also may reach fairi high percentage. In Arizona tn t writer knows of a few entire field s which were ruined, and according t t workers in the employ of the state s no fields were found last year tha Q were entirely free from the black 7 arm organism.
 The organism of black arm Q easily cultivated in the laboratory. D thrives on most of the substance : that a pathologist uses for growth bacteria, giving a yellowish colon 1 that is quite easily recognized. }. appears to be short lived in soi ! probably never holding over from on . season to the next in the ground z It is not easily killed by using for malin or other common disinfectin = reagents. especially when it is born