2 BULLETIN 1417. tJ. S. DEPARTMENT OF AGRICULTURE 



been known since 1869 and was first studied in 1888, very little is 

 known regarding its life history or the methods for its control. 



A review of the literature on root rot shows that of the many 

 pure-culture inoculations that have been made on susceptible plants 

 under many varied conditions, only a few have been reported 

 as successful. Attempts to germinate the spores of the conidial 

 stage {Phymatotrichum omnivo?mm (Shear) Duggar) have not 

 been successful. Likewise, the natural habitat and the life of the 

 fungus in the soil from year to year are rather obscure. Because 

 of its extremely erratic behavior from season to season in the field, 

 all attempts to control it by practical methods under ordinary field 

 conditions have been unsuccessful, so that root rot is still a disease 

 of major importance in the Southwest when judged by the large 

 losses caused by it from year to year. In parts of the irrigated 

 regions cultivated fields have been turned into pastures, owing to 

 the continued ravages of root rot, and farm lands known to have 

 the disease depreciate in value. 



DISTRIBUTION OF OZONIUM OMNIVORUM 



The natural distribution of Ozonium omnivorum in the United 

 States extends from the Red River Valley, which defines the bounda- 

 ries of Texas, Oklahoma, and Arkansas, south and west through most 

 of Texas and certain irrigated valleys in New Mexico and Arizona, 

 including the Colorado Valley in southeastern California. In Texas 

 the most serious losses from root rot have been reported from the 

 black waxy lands, while in New Mexico. Arizona, and v southeastern 

 California it is chiefly confined to alluvial soils. 



So far as known, root rot has never been reported in acid soils. 

 All soils of the Southwest are of a calcareous or alkaline nature. It 

 is doubtful whether this disease will ever become prevalent in the 

 cotton-growing States east of the Mississippi River. 



That Ozonium omnivorum is indigenous to this region is shown 

 by the fact that land cleared of native vegetation and planted to 

 crops susceptible to Ozonium root rot may become badly diseased. 

 A number of instances were observed where cotton planted on land 

 cleared the previous spring was badly diseased during the summer 

 of 1924. Data are also available on the prevalence of root rot on 

 land cleared of native vegetation in one of the large and recently 

 developed cotton projects on the edge of the Salt River Valley near 

 Phoenix, Ariz. Here hundreds of acres were cleared of native 

 vegetation and planted to cotton. The losses from root rot in cer- 

 tain fields were rather high during the first season. The disease has 

 persisted in these fields in subsequent years, producing a high per- 

 centage of root rot whenever planted to susceptible crops. 



Further, in a number of instances near San Antonio, Tex., 

 Ozonium strands have been found on the roots of the common mes- 

 quite (Prosoyis glandulosa Torr.) and granjeno {Momisia pallida 

 (Torr.) Planch.) both in cotton fields cleared the previous season of 

 native vegetation and under cotton plants dying from the attacks 

 of root rot. The same type of Ozonium strands was found adjacent 

 to these fields on the roots of the above-mentioned plants growing 

 under natural conditions. During the winter of 1921^25 George T. 

 Ratliffe sent to the senior writer a number of specimens of typical 



