ASCOMYCETES 



229 



summer in the United States that one unconsciously associ- 

 ates with the lilac, during that season, a grayish color. Upon 

 some hosts, however, the mycelium may form persistent patches 



(Fig- 93). 



The perithecium is generally small, with appendages averag- 

 ing 1^ times its diameter. These are colorless to light brown in 

 part, and 3 to 6 times dichotomously branched. The asci are 

 usually 3-8, each containing 4-8 relatively small (18-23 x 10- 

 12 ft) spores. 



XXVIII. POWDERY MILDEW OF GRAPE 

 Uncinula necator (Schw.) Burr. 



Bioletti, F. T. Oidium or Powdery Mildew of the Vine. Calif. Agl. Exp. 



Sta. Built. 186 : 315-352. figs. 1-17. 1907. 

 Galloway, B. T. Observations on the Development of Uncinula spiralis. 



Bot. Gaz. 20: 486-491. pi. 32-33. 1895. 

 Viala, P. Les maladies de la vigne, I.e., 2-56. pi. 1. figs. i-ig. 1893. 



This mildew is one which has long been known as an im- 

 portant fungous disease in Europe and in America. For many 

 years it was supposed that the American plant might not be the 

 same as the European, since in the latter country only the oidium 

 stage was known, that stage having been described as Oidium 

 Tuckcri Berk. It is now certain that the plant in the two coun- 

 tries is the same species. 



This species has a light mycelium, which develops on both 

 sides of the leaves and sometimes on the flower clusters. In 

 the United States it is especially abundant on the leaves in moist 

 situations during the late season. It produces a mottled and 

 slightly arched condition. During some seasons considerable in- 

 jury results to the plant. The conidia are produced in abundance 

 and the disease may be rapidly spread. The perithecia vary from 

 70 to 128 /a in diameter and are provided with a varying number 

 of appendages, usually from 10 to 20 or more, each appendage be- 

 ing from one to four times as long as the diameter of the peri- 

 thecium. These appendages are straight or slightly flexuous, 

 except as to the uncinate or incurved tip. They may be septate, 

 and amber brown in the lower half. There are usually 4-6 asci, 

 each containing 4—7 spores. 



