Diseases of Annuals 1

Alternaria flower blight of marigold. Image courtesy of Dr. Gordon Holcomb, Louisiana State University. Gray mold on marigold (Tagetes sp.) is caused by the fungus Botrytis cinerea. Wet, humid weather favors disease development. Bacterial leaf spot on zinnia (Zinnia sp.) is caused by Xanthomonas campestris. Symptoms appear as angular, reddish to dark brown spots with a prominent yellow halo that form on leaves. Rust (Puccinia oxalidis) spores being produced within pustules on the underside of wood sorrel (Oxalis sp.) leaves. This disease can develop very rapidly due to the large number of wind-borne spores produced. Galls of the root knot nematode on the roots of Ajuga reptans. Image courtesy of Dr. Gordon Holcomb, Louisiana State University.

Alternaria flower blight of marigold - closer view. Image courtesy of Dr. Gordon Holcomb, Louisiana State University. Alternaria dianthicola spores - microscopic view. Image courtesy of Dr. Gordon Holcomb, Louisiana State University. Impatiens (Impatiens sp.) roots damaged by the root knot nematode (Meloidogyne spp). Severely infected plants exhibit gall formation (knobbing) on the roots, as seen in this image. The plant appears wilted, chlorotic and stunted, resulting in overall reduced plant growth. Botrytis blight (tulip fire) is a fungal disease of tulips caused by Botrytis tulipae. Symptoms include light tan patches on the flowers, which can coalesce to blight entire flowers or leaves. Image courtesy of Clemson University - USDA Cooperative Extension. Begonia (Begonia sp.) infected with tomato spotted wilt virus (TSWV). Note the chlorotic areas on the leaf. This virus is spread by thrips. TSWV is acquired only by the thrips larvae, which in some cases can take only 15 minutes. The virus is vectored almost exclusively by adult thrips.