Strains of pathogens that mildly harm, but do not kill, the host plant are termed

Avirulent is the strains of pathogens that only mildly harm but do not kill the host plant are termed. Plant pathogenic bacteria cause many serious diseases of plants throughout the world but fewer than fungi or viruses, and they cause relatively less damage and economic cost.

the dark variety, as they are not visible on the dark tree barks

explanation:

we would need to see the picture to answer it