Diseases

Angular Leaf Spot on Zinnia

Angular leaf spot caused by Xanthomonas campestris pv. zinniae. Bacterial diseases are most aggressive in hot weather as bacterial growth is favored by high temperatures. Overhead or sprinkler irrigation aggravates bacterial blights. Xanthomonas species are carried on seed; start with pathogen-free seed. If treated seed is not available-seed can be treated with a dilute bleach solution (10%) for 15 to 30 minutes or hot water. Be sure to test a small batch of seed for sensitivity to these treatments.

Bacterial Leaf Spot on Garden Mums

Bacterial leaf spot, caused by Pseudomonas cichorii often occurs during hot humid weather. This disease tends to be problematic during years of heavy rains like this year, or where overhead watering is practiced. Plants with this disease have large black spots concentrated at the base of the plant. The spots often begin at the leaf margin but may also occur randomly. The spots are soft when tissue is wet and sunken and brittle when leaves dry. From the leaf, the bacterium can move through the petiole and into the stem resulting in a canker.

Downy Mildews

Downy mildews symptoms appear very rapidly and tend to be much more difficult to control than powdery mildew. Conditions that promote leaf wetness, such as high relative humidity, overhead irrigation and close spacing, favor this disease. With a hand lens, closely examine the bloom of sporangia (microscopic stalks bearing spores) on the underside of the oldest leaves. (They will look like many tiny branched trees, each bearing tiny lemons). This can help you distinguish downy mildew from powdery mildew.

Fusarium Wilt on Garden Mums

Fusarium wilt caused by Fusarium oxysporum f.sp. chrysamthemi is a vascular disease that develops within stems. The fungus is soil-borne and after entering the plant through the roots, invades the stem plugging water conducting tissue (xylem) with mycelium and spores. Fusarium wilt symptoms are often confused with root rot but plants infected with Fusarium generally wilt in sectors (one side) and roots often appear healthy. Root rot diseases usually result in the entire plant wilting.

Fusarium Wilt and Corm Rot of Cyclamen

Fusarium wilt and corm rot of cyclamen is caused by Fusarium oxysporum f.sp.cyclaminis. Symptoms appear at any stage of plant growth and are dependent on both inoculum levels and growing conditions. The leaves exhibit yellow patches and wilt one after another until the entire plant collapses. The most distinctive symptom is the purple, reddish-brown to black vascular discoloration of the corm, which remains firm unless it is invaded by secondary bacteria. Root rot may also occur.

Pythium Root Rot on Poinsettias

When Pythium spp. shows up late in the crop cycle, there are few options for salvaging the crop. Once the fungus is managed, it takes time for roots to re-grow, and there may not be enough time for plants to recover prior to sale. Signs of infected plants are wilting and stunting. Roots are soft and decayed, sometimes extending up into the stem where it causes a canker. Looking closely, the rotted outer covering of the root slips from the central core. There are different species of of Pythium that can cause problems on poinsettias.

Soil Testing and Disease Diagnostic Labs

With the spring growing season upon us, growers are reminded that floriculture diagnostic labs and soil test labs are available through Universities to help prevent and solve problems. Test growing media early and often to maintain proper pH and fertility. Use a diagnostic lab for early, accurate diagnosis of plant diseases. Catching problems early will prevent the misuse of pesticides, save you money and reduce crop loss. Below is a list of University laboratories in Massachusetts, Connecticut and Rhode Island.

Massachusetts

Hosta virus x identified in Massachusetts

HVX is a potexvirus, and, currently, hostas are the only known host. In October, Massachusetts reported its first case of HVX. The varieties Sum & Substance, Striptease, and Gold Edger tested positive when sent to Agdia for ELISA testing. The nursery destroyed the infected plants. The most common symptoms of hosta virus x are mottled or crinkled leaves. On plants with gold leaves a green mottling, especially along the vein, indicates viral infection. There are several hosta viruses; so, virus testing is the only way to determine which virus your plants have.

Pythium Root Rot

Stunted growth and wilted plants are common above-ground symptoms caused by Pythium root rot. To examine plants, remove plants from pots and examine roots. Healthy roots are white and firm; decayed roots may be dark colored and the rotted outer covering of the root slips from the central core.

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