May IS, 1920 Halo-Blight of Oats 147 



injured with a scalpel or drawn between the fingers to rub off the bloom. 

 Bundles of control plants were treated in the same manner and sprayed 

 with sterile water. All inoculated and control plants were covered with 

 glassine bags for 48 hours, as stated above. Characteristic halo lesions 

 appeared on all the varieties inoculated except Wisconsin No. 4. Only 

 uninjured plants of this variety were inoculated. Five other varieties 

 (No. 22, 25, 49, 52, and 62) showed no lesions on uninjured plants, but 

 all varieties showed fairly abundant spotting of leaves and sheaths of 

 plants which had had the bloom removed or had been cut with a scalpel. 

 Some of these leaves were almost entirely yellowed with lesions. On 

 6 varieties lesions appeared on uninjured plants, but the lesions were not 

 nearly so abundant as on injured leaves and panicles. Wisconsin No. 7 

 was the only variety in which the panicles were entirely out of the 

 sheaths. In this variety every spikelet of the injured panicles showed 

 halo lesions which stood out as oval yellow spots on the glumes. About 

 half of the spikelets in these panicles were not filled out. Spikelets of 

 untreated panicles of the same variety were also poorly filled out. Under 

 favorable conditions the panicles appear to be just as susceptible to halo- 

 blight as the leaves. Wisconsin No. 14 also showed heavy spotting of 

 injured panicles. Uninjured spikelets of two varieties were halo-spotted 

 when the suspension was sprayed into the unopened sheath. 



Though none of the controls showed any halo lesions, both water- 

 sprayed controls and inoculated plants showed considerable sterility, 

 amounting to from one-fifth to one-half of the spikelets in a panicle. 

 Untreated heads of the same varieties and in the same plots showed either 

 no sterility at all or only traces at the base of the panicle. This sterility 

 was particularly abundant when either the water suspension or sterile 

 water was sprayed into unopened sheaths or sheaths just opening at the 

 top. The fact that both controls and inoculated plants showed the same 

 amounts of sterility would indicate that the sterility was not due to the 

 effects of the organism. Excessive moisture around the developing 

 spikelets while these were still inclosed within the sheath offers the most 

 plausible explanation for this sterility. In the same way heavy rains at 

 the time oat fields are heading out probably account for the sterility 

 commonly observed in oat fields. This set of field inoculations has led 

 to the following conclusions : 



1. Leaves and panicles of oat plants approaching maturity are sus- 

 ceptible to halo infection under favorable conditions. 



2. Infection takes place more readily on injured than on uninjured 

 parts of the plants. 



3. Some varieties are more susceptible to infection than others. Green- 

 house inoculations on young plants also led to this conclusion. 



4. Although both natural and artificial halo infection may occur on 

 heads, these infections are not responsible for the blasting of oat heads. 

 Sterility is due probably to physiological rather than pathological con- 

 ditions. 



