42 BULLETIN 727, U. S. DEPARTMENT OP AGRICULTURE. 



perses the spores widely through the field and causes a marked 

 downhill spread of infection. 



MOISTURE CONDITIONS FAVORING INFECTION. 



Judging from the evidence already presented under the considera- 

 tion of water dissemination, it seems quite evident that the very 

 agency of dissemination in that case furnishes the proper condition 

 for spore germination and subsequent host infection. In view of 

 the even-aged "crops" of new lesions following each rain, it appears 

 that infection with anthracnose generally occurs while the plants 

 are still wet from a rain. 



However, it was pointed out that some incipient infections are often 

 to be found after periods of prolonged drought, and this was attrib- 

 uted to delayed penetration by spores already present. Whether 

 these spores were present as such or as appressoria is not known, 

 although it seems quite probable that the latter may represent a 

 resistant resting stage. 



Now, as to the conditions favoring such cases of delayed infection, 

 it is of importance to recognize that during August in Wisconsin 

 heavy dews occur at night and cucumber foliage is heavily coated 

 with moisture. Observation has shown that this water of condensa- 

 tion remains in drops on the lower epidermis of cucumber leaves, 

 while it spreads over and thoroughly wets the upper epidermis. This 

 daily wetting of the leaves no doubt affords favorable conditions for 

 germination of the spores or penetration from the appressoria. 



Furthermore, there is a possibility that infection may be facilitated 

 by guttation water. Large drops of guttation water often collect 

 about the margins of cucumber leaves when evaporation is retarded, 

 as, for example, in the case of plants under cloth cages. This water 

 is sometimes exuded in such quantities as to run down over the epi- 

 dermis of the leaf. Marginal anthracnose lesions are common. Two 

 samples of guttation water were collected from caged plants, Septem- 

 ber 6, 1916, and tested as a medium for spore germination. While 

 only a very low percentage of germination was obtained, proof was 

 afforded that the liquid was not toxic. 



Examination of watermelon fruits in the South showed the fre- 

 quent occurrence of dense masses of incipient anthracnose lesions, 

 or u pimples," about the periphery of the bleached area where the 

 melon was in contact with the soil in the field. Subsequent exami- 

 nation of fruits as they lay in the field revealed that this region of 

 their surface was coated with a layer of minute droplets of condensa- 

 tion water, while the part in actual contact with the soil was thor- 

 oughly wetted. This condition obtained even during the heat of the 

 day. It seems quite probable that the exposed band of condensation 

 moisture on the under side of the fruit would afford conditions of 

 moisture and oxygen supply very favorable to spore germination. 



