294 



Bulletin 255. 



ing and previous, the fungus has been spreading and becoming established 

 on the stems and leaves, and it now attacks the young and succulent 

 pods. With their tender growing tissue full of water and food materials, 

 these pods ofTer the best conditions for the growth and development of 

 the parasite. Spores from the spots on the leaves and stems fall on the 

 pods, where, in the presence of moisture and the high summer temperature, 

 they germinate, forming a little sprout or germ-tube, which penetrates 

 the tender skin of the pod (Fig. 221) and, branching in the juicy tissues, 



gives rise to an anthracnose 

 canker. These first appear 

 as little brown or rusty spots 

 which enlarge and darken 

 until nearly or quite black. 

 The dead tissue dries and 

 settles, causing a little pit or 

 sunken place in the pod. In 

 the center of the spot the 

 spores of the fungus are now 

 produced in great abundance. 

 They ooze out and pile up, 

 forming little pink masses 

 easily seen with the naked 

 eye (see Figure 217). These 

 masses of spores are held to- 

 gether by a kind of glue or 

 mucilage which, when dry, 

 sticks them tightly to the 

 spot. When a drop of rain 

 or dew falls on the spot the 

 mucilage is at once dissolved, 

 and the spores are set free in the water. At this time any disturb- 

 ance of the bean plants will scatter these spores in the flying drops of 

 water. In this way they reach health}^ plants near by. This explains 

 why beans should not be cultivated or handled in the early morning 

 while the dew is still on them or directly after a shower. The spores of 

 the anthracnose fungus are scattered only when they are wet. This 

 will also explain why a warm rainy season is so favorable to the de- 

 velopment of the fungus. The spores require moisture in which to be 

 distributed and in which to germinate. A relatively high temperature 

 is also most favorable to the disease. The spores are produced in un- 

 limited numbers in the spots on the pod. Fig. 222 shows the spores 



Fig. 222. — Spore of the anthracnose fungus 

 taken on the point of a needle and placed in a 

 drop of water. Magnified about §5 times. The 

 large blur is a mass of the spores from which 

 others have become detached and scattered about 

 in the water. 



