IX. MCCULLOCH'S CAULIFLOWER SPOT 



Type. — This is a disease of cauliflower and cabbage, char- 

 acterized by a copious stomatal blotching, spotting or specking 

 of the green leaves, both on the veins and in the parenchyma. 

 When the midrib and the veins are attacked early and seriously, 

 the leaves become variously puckered and distorted (see also 

 Nos. VIII and XI). Seen by reflected light, the spots are at 

 first water-soaked, then brownish to purplish gray, but by trans- 

 mitted light they appear thin and almost colorless in the center, 

 with a dark border. The spots on cabbage are darker than those 

 on the cauliflower. The individual spots, which occur in great 

 numbers, are usually small (mere points to areas 1 to 3 milli- 

 meters in diameter, seldom larger), and are circular when quite 

 small but soon become more or less irregularly angled, owing 

 to limiting veins. By coalescence, elongated, irregular, and 

 much larger spots occur, making the leaves look ragged. The 

 badly spotted leaves also turn yellow and fall off (3 to 5 weeks 

 after infection). 



The disease was not observed naturally infecting the bleached 

 flowering parts of the cauliflower in the Virginia material, nor 

 was it obtained by spraying such parts, except sparingly on 

 some of the larger flowerstalks, but later was obtained on cauli- 

 flower heads from Florida and produced on the cauliflower heads 

 in one of our hothouses by pure culture inoculations. Probably 

 the stomata do not function as readily on the bleached parts as 

 on the green parts and consequently are less likely to be entered 

 by the bacteria. 



All of the infections, so far as observed, are stomatal, each 

 small round spot having a single stoma in its center, below 

 which is a bacterial pocket. By spraying water-suspensions of 

 young agar-streak cultures upon cauliflower plants and cabbage 

 plants, numerous infections were obtained (Fig. 231) but mostly 

 on the under surface of the leaves. When the infectious spray 

 was confined to the upper surface of the leaves very few spots 



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