300 University of California Publications in Agricultural Sciences [Vol.3 



common. In addition we obtained one strain possessing the aseigerous 

 stage, which of course classified it in the genus Pleospora. Several 

 Macrosporium strains were also isolated. 



Considerable effort was made to identify the Alternaria strain 

 so commonly found, but we have been unable to satisfy ourselves 

 thoroughly in this regard. While the literature is indeed voluminous, 

 there is apparently no reliable monograph of the genus. Recently, 

 however, there has appeared a critical study of the taxonomic char- 

 acters of the genus. 35 The genus Alternaria is one of the most uni- 

 versally distributed of the common forms of the Fungi Imperfecta. 

 It embraces about fifty species, although it has been shown by Elliott 

 that a large number of the species of the closely related genus Macro- 

 sporium really belongs to the genus Alternaria. Among these species we 

 find active parasites as A. solani (E. and M.) J. and G., weak or facul- 

 tative parasites as A. citri Ellis and Pierce, and saprophytes as A. 

 tenuis Nees. Certain species have already been secured in the perfect 

 or aseigerous stage which has always proved to be Pleospora. Since 

 the strain under consideration was uniformly obtained from oranges in 

 a district where black rot is common it is probably the same form found 

 by Pierce and called Alternaria citri. We were unable to find the 

 original description by him, which does not seem to have been pub- 

 lished. However, after examining the literature and drawings of 

 Alternaria citri, particularly as given by Rudolph, 36 we feel reasonably 

 sure that we are dealing with Alternaria citri E. and P. and throughout 

 the remainder of the discussion we shall proceed on that assumption. 



The spores of Alternaria citri are borne in long chains (pi. 36), 

 which readily break up, allowing the spores to float away in the air. 

 It seemed important to determine whether the infection of oranges was 

 accomplished by spores borne by the air or those carried by honey- 

 bees and other insects. The following methods were employed. Petri 

 dishes containing Shear 's corn meal agar were exposed for five minutes 

 in different localities. After a few days had elapsed in order to allow 

 the various bacteria, molds and other fungi to assume colony form 

 and the Alternaria, if present, to produce spores, the dishes being 

 inverted were placed under the low power of the microscope and the 

 colonies of Alternaria easily distinguished and counted. On account 

 of the length of the spore chains and certain other morphological 



35 Elliott, J. A., Taxonomic Characters of the Genera Alternaria and Macro- 

 sporum, Am. Jour. Bot., vol. 4 (1917), pp. 439-76. 



36 A New Leaf-Spot Disease of Cherries, Phytopathology, vol. 7 (1917), pp. 

 188-97. 



