This page will give images and information on some of the diseases that may afflect your zinnias.
Alteraria leaf blight:
DESCRIPTION: Alternaria is a fungus which causes leaf spotting. Leaf spots progress from older leaves to newer. Spots are circular, target-shaped, and often surrounded by a slight yellow area. Entire leaves may die and drop from the plant. They may also be found around stem ends on fruits and on stem.
LIFE CYCLE: Alternaria survives in plant debris and may be spread by insects or wind, and by rain/irrigation. Spores germinate in several hours during high humidity conditions. Infection occurs through natural plant openings if water remains on plant tissue for more than a couple of hours. After 2-3 days, first symptoms appear. Optimum temperatures for infection are 60-75° F, high humidity, and low fertility.
Cultural Control: This disease is spread by splashing water and by walking through plants when wet. Keep water off leaves. Water with soaker hoses or drip irrigation. Use 3 – 4 year rotations between susceptible crops. Maintain adequate nitrogen levels. Remove and destroy infected leaves.
Chemical Control: Sulfur and copper can be sprayed when temperatures are between 55 and 85° F and weather is wet, to protect leaves from infection. These “least toxic” options are less effective when overhead irrigation is applied. Missoula County Extension Service
Bacterial Leaf Spot
General Recommendations: Bacteria that cause leaf and flower spots, blights and fruit rots are ever-present in the environment. They survive in diseased plant debris from one year to the next and are transferred to new plants by wind, splashing rain, flowing water, contaminated soil and insects. Since bacteria typically require a wound through which to enter and infect a plant part, anything that can cause injury to plants, including abrasion by blowing sand, hail, pruning cuts and other tools, are commonly associated with bacterial infections. In some cases, natural openings in the leaves (stomata and hydrathodes), flowers (nectaries), and stems (lenticils) can serve as entry points for bacteria. Symptoms of bacterial infections vary with the host. A general symptom that indicates that bacterium is involved is the presence of a yellow halo surrounding a tan to dark brown lesion. This symptom is not always present when bacterial spots and blights occur, but it is a positive indication when present. Systemic infection can also occur when roots are damaged and bacteria enter, traveling up the stem in the water- or nutrient- conducting tissue. Leaf spots and blights result from a more localized infection. Rots occur when fleshy tissue is broken down by the action of rapid bacterial development. The progressive deterioration of rotting tissue typically leads to offensive odors which are also diagnostic of a bacterial infection.
Control of Bacterial Spots, Blights and Rots:
1. Clean up diseased debris. With a few exceptions, bacterial plant pathogens survive from year to year because they are protected in plant debris, including leaves, stem and roots that they infest. Since the ability of bacterial pathogens to move from place to place is limited, eliminating diseased debris is a primary factor in reducing the incidence of disease. This debris should be removed from the garden and buried or discarded. In addition, turning the soil over to bury any debris that cannot be picked up is also an important step in reducing the chance of disease spread. Buried debris will eventually decompose and decrease the viable population of bacteria. Composting infested plant material is not advised.
2. Remove infested plant parts. Bacterial infections, discovered during the growing season, should be removed from the plant. Use clean tools as the use of contaminated tools can exacerbate disease spread. Disinfect tools between cuts by dipping them in rubbing alcohol or a solution of 1 part bleach to 4 parts water. When finished, clean the tools and wipe with an oiled rag to lubricate the parts and discourage rust. Avoid extensive pruning during wet or humid weather when the bacteria can be easily transmitted to and enter wounds. When seeking to reduce the spread of bacterial diseases, such as fireblight, Mid summer or fall pruning is preferred to spring pruning. An occasional affected branch on an otherwise clean plant can be pruned almost anytime. Prune 6 -12 inches behind the affected area.
3. Mulch. Since infested debris must be exposed to splashing rain in order for bacteria to be moved to a new infection site covering the soil around the plant with a layer of leaf mold or compost which buries the debris will help to prevent the spread of bacteria to new plants. Mulch also buffers the soil from extreme moisture loss, reducing the amount of stress during drought conditions.
4. Avoid overhead watering. Splashing rain or water is the primary vehicle that transfers bacterial pathogens around a plant or to a new host plant. Reducing the amount of overhead watering lessens the chances that the bacterial inoculum will be spread both from plant part to plant part and from the soil, where infested debris may reside, to the plant.
5. Rotate crops. Since bacteria populations tend to build up on diseased debris if the same plants are grown in the identical location year after year rotate unrelated vegetable and flowers in and out of a garden planting site each year.
6. Apply a bactericide. Except for copper and streptomycin sprays, which can not be used on all plants, there are few materials available that are effective in controlling bacterial diseases. And, even these bactericides provide limited insurance when a plant has open wounds and are ineffective after infection has occurred. Consequently, disease prevention is the most important initial step.
7. Use clean seed. Some bacterial pathogens can be introduced via seed where they reside either as contaminants on the outside of the seed coat or in wounds. Discard shriveled and discolored seed. Seed treatments have limited value.
8. Select resistant cultivars. Resistance to bacterial diseases is a common goal of vegetable and herbaceous plant breeders. Always select resistant cultivars when they are available, especially when growing plants in humid environments.
Powdery mildew is one of the most widespread and easily recognized plant diseases. Powdery mildews are most severe when the weather is warm and dry, and they affect virtually all kinds of plants: cereals and grasses, vegetables, flowers, weeds, shrubs, fruit trees, and broad-leaved shade and forest trees. Many plants have been developed to be resistant to or tolerant of powdery mildew. Succulent tissue is the most susceptible to infection.
Even though there are several types of powdery mildew fungi, they all produce similar symptoms on plants. Powdery mildews are characterized by spots or patches of white to grayish, talcum powder-like growth. Tiny, pinhead-sized, spherical fruiting structures–first white, then yellow-brown and finally black–may be present singly or in a group. These are the cleistothecia or overwintering bodies of the fungus. The disease is most commonly observed on the upper side of the leaves. It also affects the bottom sides of leaves, young stems, buds, flowers and young fruit. Infected leaves may become distorted, turn yellow with small patches of green, and fall prematurely. Infected buds may fail to open.
The severity of the disease depends on many factors: variety of the host plant, age and condition of the plant and weather conditions during the growing season. Powdery mildews are severe in warm, dry weather because, unlike most fungi, powdery mildew require a wet leaf surface for infection to occur. However, the relative humidity of the air does need to be high for spore germination. Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Incidence of infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces are wet (e.g., in a rain shower). Young, succulent growth is usually more susceptible than older plant tissues. Powdery mildew can seriously impact yield on flowering crops such as squash, pumpkins, cyclamen and reiger begonia, but on other plants such as lilac and oak, the mildew is unsightly but does not severely harm the plant.
Powdery mildews are host specific: they cannot survive without the proper host plant. For example, the species Uncinula necator, which causes powdery mildew on grape and linden, does not attack lilac. Similarly, Microsphaea alni affects elm, catalpa, lilac and oak but not turfgrass.
Powdery mildews produce mycelium (fungal threads) that grow only on the surface of the plant. They never invade the tissues themselves. The fungi feed by sending haustoria, or root-like structures, into the epidermal (top) cells of the plant. The fungi overwinter on plant debris as cleistothecia or mycelium. In the spring, the cleistothecia produce spores that are moved to susceptible host tissue by splashing raindrops, wind or insects.
Several practices will reduce or prevent powdery mildews. Many plants, such as roses, vegetables and Kentucky bluegrass, are developed to be resistant or tolerant to powdery mildew. Use resistant varieties whenever possible.
Once the disease becomes a problem:
• Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue (which is more susceptible to infection).
• Avoid overhead watering to help reduce the relative humidity or water in the early morning to let the tissue dry as soon as possible.
• Remove and destroy all infected plant parts (leaves, etc.). For infected vegetables and other annuals, remove as much of the plant and its debris in the fall. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus.
• Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection.
• An alternative nontoxic control for mildew is baking soda combined with a lightweight horticultural oil. Researchers at the University of Rhode Island have confirmed that a combination of 1 tablespoon baking soda plus 2.5 tablespoons oil in 1 gallon of water is effective against powdery mildew on roses. Use of this combination on other crops is still experimental.