A technique for inoculating and measuring the relative resistance to Colletotrichum lagenarium of cucurbits is described.

Typical symptoms of anthracnose are given for the more common hosts, and symptoms are briefly described for each new host. Symptoms of anthracnose on cantaloupe are contrasted with symptoms of other cantaloupe leaf spot diseases. A previously unrecorded symptom is reported for watermelon on the runners near the crowns of the plants.

Temperatures of 15° to 17° C. or less during the 14-16-hour period in the moist chamber resulted in no infection. Within the temperature range (20-30° C.) infection takes place. The manifestation of symptoms is retarded 24 hours by lowering the temperature 10° C.

The host range of C. lagenarium has been extended to include the following susceptible cucurbits: Apodanthera undulata., Cucurbita foetidissima, C. texana, Ibervillia tenuisecta, Luffa sp., and Sicyos parviflorus.

From the standpoint of relative resistance, the genus Cucurbita may be said to be the most resistant genus studied, while Cucumis and Citrullus are the most susceptible.

Slight differences in resistance and susceptibility were found in commercial watermelon, cantaloupe and cucumber varieties, but in no case was the resistance sufficient to be of commercial importance.

The comparative resistance or susceptibility is given for the following cucurbits:

Watermelon

71 Commercial American varieties (163 seed lots).

6 Citron varieties.

165 Foreign varieties from 15 countries.

407 Kleckley Sweet selections.

187 Conqueror selections.

Cantaloupe

32 Commercial American varieties.

26 Foreign varieties from five countries.

81 Selections.

Cucumber

18 Commercial varieties.

Cucurbita spp.

21 Commercial varieties

Three edible, non-commercial African watermelon varieties were found which were highly resistant to anthracnose but susceptible to wilt. Crosses of these varieties, Africa 8, Africa 9 and Africa 13 were made on Iowa Belie, Iowa King and a few miscellaneous watermelon varieties. Resistance to anthracnose was dominant to susceptibility and segregation in the F₂ and in the F₁ backcross generation proved that only a single factor pair for resistance and susceptibility was involved. From these crosses strains of watermelon have been developed which are resistant to anthracnose and wilt.