106 U.S. DEPT. OF AGRICULTURE MISC. PUB. 939 



J. scopulorum Sarg. 

 /. squamata Lamb. ( Planted in U.S. ) 

 /. virginiana L. 

 Thuja occidentalis L. 

 T. plicata Donn 



T. orientalis L. (Planted in U.S.) 

 Literature : 



Hahn, Glenn Gardner. Phomopsis juniperovora and closely re- 

 lated strains on conifers. Phytopathology 16: 899-914. 1926. 



Life-history studies of the species of Phomopsis occurring 



on conifers. Part 1. Trans. British Mycol. Soc. 15 : 32-93. 1930. 

 Taxonomy, distribution and pathology of Phomopsis oc- 





culta and P. juniverovora. Mycologia 35 : 112-129. 1943. 



Phymatotrichum Root Rot 



Ernest Wright 



Oregon Forest Research Center, Corvallis, Oregon 



Phymatotrichum omnivorum (Shear) Duggar — Phymatotrichum 

 root rot, also known as Texas or cotton root rot — is a deadly disease 

 of dicotyledonous plants. Monocots are immune. This root rot 

 overwinters in the soil on live roots and by sclerotia. Growth through 

 the soil is very limited; normal spread is by root contacts. Shear 

 designated the perfect stage as a Hydnum. The mycelial stage is an 

 Ozonium and its conidial stage Phymatotrichum. The Ozonium form 

 is observed readily on infected roots as fuzzy, yellowish, acicular 

 hyphae. P. B. Streets described and illustrated these various stages. 



Following summer rains, the conidial stage may appear above 

 ground in the form of spore mats. The spores are smooth, globose, 

 averaging 4.8 to 5.5/* in diameter, or sometimes ovate with a measure- 

 ment of 5 to 6 by 6 to 8/x. Spores do not germinate readily and fail 

 to produce prolific hyphae. Spore mats, however, serve to designate 

 the presence of this root rot. Besides wilted cotton or alfalfa and 

 other agricultural crops, certain weeds such as ragweed and horse- 

 nettle serve as indicators of the presence of root rot. Diseased plants 

 seldom give advance indications of infection, usually wilt suddenly 

 and die. Root rot infected plants typically occur in roughly circular 

 spots. Trees and shrubs at first may show a reduction in growth and 

 vigor. Leaves of infected trees assume a yellow to bronze coloration 

 and fall prematurely, or gradually drop off giving the crown a' thin, 

 sickly appearance. Resinosis is usually lacking. 



Range : Phymatotrichum omnivorum is indigenous to the southwest- 

 ern part of the United States and northern Mexico, extending 

 from extreme eastern Texas into New Mexico, Arizona and south- 

 eastern California. It also has been found in Arkansas (SE. cor- 

 ner), Oklahoma (in the extreme south along the Red River), 

 southeastern Nevada, and southwestern Utah. Reports of this 

 root rot elsewhere in the world have not been verified. 



Hosts: All dicotyledonous plants tested are susceptible in varying de- 

 gree. Trees have been graded in susceptibility by Streets, Tau- 

 benhaus and Ezekiel, and more recently by Wright and Wells. A 

 comparison of these ratings under different field conditions is 

 given in Table 1. 





