Fungus Control for Dahlias and the Garden

After a few years growing dahlias, many gardeners invariably have questions concerning fungus. This usually starts with them simply asking, “how can I prevent my tubers from rotting”, realizing in most cases the reason for the decay is caused by some form of root rot. Often they see less innocuous forms of fungus on the surface of the tubers in the form of molds (Botrytis or gray mold). Many times the presence of fungal diseases is overlooked by even the most experienced of growers. Plants with root rot disease sometimes mimic other plant problems like nutrient deficiency or heat and water stress. Plants treated with an appropriate fungicide will have larger root systems, develop faster and have improved yields. The increased root mass enables greater uptake of nutrients and water resulting in larger and better quality blooms.

The late Phil Traff attributed larger dahlia tuber clumps to his treatment with the fungicide Terraclor (pp ??, Dahlias of Today 198?). After more experience growing dahlias many growers begin to recognize other symptoms of fungus appearing on the plant, white patches (powdery mildew) on the green leafy growth and perhaps small dark beads (Sclerotinia) along the stems. In the case of young cuttings or seedlings they may see signs of “damping off” caused by root pathogens, especially the oomycetes commonly known as “water molds” (Pythium and Phytophthora). Occasionally the fungi Fusarium, and/or Rhizoctonia will contribute to root and stem rots; Fusarium, also being the most common cause of “dry rot” in tubers. All of these diseases can be prevented with fungicides, either systemically or topically. Bear in mind, to prevent most forms of root rot internally, one must use a systemic.

However, fungicides will not be totally effective in eliminating all fungi from soil and will not cure root rot once it begins. The best cure is prevention, removing the conditions that harbor or encourage the formation of fungi. Since many fungal diseases are difficult to control once established, every effort should be directed toward prevention. Sanitation is the first important step, disinfect all pots, trays and tools with 10% bleach (sodium hypochlorite), or bromine solution. Remove dead or dying tissue from plants, clean refuse from the greenhouse and around the garden. Reduce the conditions that favor “damping-off” which frequently attacks young seedlings/plants (causing them to fall over and die):

  • excessive soil moisture and excessive overhead misting.
  • overfertilization and build up of insoluble salts
  • low soil temperature before germination (below 68 degrees F)
  • high soil temperatures after emergence (above 77 degrees F)
  • overcrowded flats or seedbeds

But, no matter how careful a grower is, disease caused by soil-borne pathogens can still occur and treatment with fungicides becomes necessary.

There are two general types of fungicides, contact or protectant, and systemic. Protectant types inhibit germination of spores (small reproductive bodies) and prevent infection if applied topically to healthy tissue surface prior to infection. Once infection has occurred and fungus becomes established within the tissue, a protectant type fungicide will no longer be effective. Systemic fungicides are absorbed by the leaf tissues or root system and remain local or are translocated throughout the plant. They will not only prevent infection, but often will cure and eradicate the fungus established in the plant tissue. Because of their activity, systemic fungicides generally require one well timed application, but are more likely to cause resistance if used exclusively. Whereas, contact fungicides usually require multiple applications spaced 7-10 days apart and in general have a broad spectrum of control. When using fungicides it is important to alternate different classes (and also systemic and contact) in order to avoid populations resistant to some fungicides, a common problem, particularly with Botrytis. In general, fungi resistant to one chemical class are resistant to all other chemicals in the family. Do not rely entirely on one chemical class for control. Alternate or mix a systemic with a contact fungicide. When tank mixing two fungicides for a single spray use each at the label rate; do not reduce the concentration of either. Not all are compatible with other chemicals, and some are likely to cause plant damage if used incorrectly. ALWAYS READ THE INSTRUCTIONS/LABEL BEFORE USING ANY FUNGICIDE.

Many fungicides can cause phytotoxicity or damage plants. Test a small area before treating all of the plants. It is important to keep in mind that no single fungicide will kill or inhibit the growth of all fungi. However fungicides are typically effective against “groups” of fungi and therefore a different chemical is not needed for each and every fungus. Unfortunately, since fungi start out as microscopic, (only their colonies or structures are visible), they are often difficult to identify positively without proper equipment and training. I have narrowed the fungus listed to the most common or likely to occur in a dahlia patch or around the garden (on vegetables, roses and other ornamentals). Not all fungicides listed in the table are labeled for dahlias and not all fungi listed predominately affect dahlias. Each fungicide listed is specific for certain plants and fungus pathogens. Unfortunately, many of the fungicides listed in the table for treatment of specific diseases are not readily available to the home gardener. None are highly toxic or restricted use, but they are typically packaged in large quantities (and prices) more suitable for commercial growers. This is regrettable, because many of the fungicides commonly available at local garden shops do not treat the myriad of fungal diseases one is likely to encounter. (Most of these fungicides can be purchased through horticultural or agricultural supply companies, e.g. E.C. Geiger, Brighton By-Products, D&L Grower Supplies, BFG -Burton Flower & Garden, etc). Check labels and ingredients carefully, many of the active ingredients are labeled under a variety of trade names. Also, check the concentration of active ingredients, which can vary in different formulations (liquid, powder, granules, etc). Finally, be sure to only use a fungicide on plants listed on it’s label; it is illegal otherwise.

There are many classes of fungicides ranging from the older controversial EDBCs to the newer biological controls. Each class varies in its mode of action, spectrum, physical properties in the plant, and its ability to withstand resistance. As new fungicides are developed older ones are replaced. We can expect to see a new class of fungicides, called the “Strobilurin” group, becoming increasing available for home use. These highly active fungicides are considered “fast track” or “green” compounds by the EPA. In general, they are very safe to the applicator and to the environment. They are used at relatively low rates (0.2-0.25 lb. of active ingredient per acre) and can control all four groups of fungi. Because they degrade fairly rapidly they don’t contaminate soil or ground water or harm wildlife. The Strobilurin compounds have a new and novel mode of action for control of fungi. They work by inhibiting the mitochondrial electron transport system in plant pathogens, but not in other living entities. Thus they are both effective and selective. Chemicals in this group include kresoxym-methyl, azoxystrobin, trifloxystrobin, and others.

Powdery Mildew (Erysiphe cichoracearum): – is characterized by a fine white (or light gray) talcum-like appearance on leaf surfaces (tops or bottoms) and along stems. Powdery mildew seldom kills the plant, but its infection can severely limit the plant’s health and growth. Infection peaks under warm days with low humidity, and cool nights with high humidity conditions (and is in fact inhibited by moisture on the leaves).

Rhizoctonia (solani)(brown patch): – is a soil borne fungus known to cause root rots, stem rots and “damping-off”. It is favored by warm, moderately moist soil conditions. Once the fungus begins to grow in the plant, the infected areas decay rapidly. Under high humidity the fungus will quickly spread to neighboring leaves. The leaf spots produced by Rhizoctonia are very peculiar to this type of fungus. They may be as large as 1/4 inch and are oval or peanut-shaped. The most distinct feature of the spots is the clearly defined, light-colored outline. Inside this clearly defined outline will be an area of mottled discoloration marked by “zones,” i.e., spots within spots. This discolored area inside the outline will range from light green to yellow, especially in the early stages of the fungus. As the fungus progresses, however, the spots will begin to turn more brown in color. Symptoms are wilt during midday and stem rot at the soil line with brown to reddish lesions. Soil will often cling to the cankered areas of the plant when removed from the soil because of the coarse brown mycelium (body of the fungus). Rhizoctonia also favors stress conditions including soil compaction and excessive drying and rewetting of the soil medium causing the roots to crack and decay.

Pythium: – is a common fungus found in soil, sand and water that is the major cause of root rots. It is one of the fungi in the oomycete class often referred to as “water molds”. It is easily introduced in soilless mixtures with dirty tools, pots or flats. Almost all plants are susceptible to Pythium root rot, which can rot the base of unrooted cuttings. In general, Pythium is restricted to young or succulent plants. Conditions that favor root rot pathogens are high soil moisture, high soluble salts and poor aeration. Symptoms of infection include stunted plants that wilt at midday and recover at night; brown tissue on the outer portion of the root easily pulls off leaving a bare strand of vascular tissue exposed. Rot will often proceed up the stem eventually causing the plant to yellow and die. Pythium’s white cobwebby growth (mycelium) can often be seen on dew-covered grass in early morning.

Phytophthora (spp) – rot is related to Pythium and as one of the fungi in the oomycete class, has similar symptoms and treatments as Pythium. A soil-borne fungus, it is a common contributor to seed rot and pre-emergent and post-emergent “damping-off”. This disease was the cause of the Irish potato famine (1843-47). The fungus can be airborne, waterborne and seed-borne. The disease can first appear as water-soaked areas on the leaves of plants, with a white mold on the underside, areas around the lesions on the stem shrivel and pinch the seedling off at the base.

Downy Mildew: – (not related to powdery mildew) is in the same class of fungi as Pythium and Phytophthora (oomyctes) and is considered a “water mold”. The first symptom is green to yellow spots on the upper surface of the leaf. When the leaf if overturned a downy white to gray growth (tuft) may be seen opposite the upper area of sporulation. Confirmation of downy mildew infection must be made with a microscope. Downy mildew infection favors high humidity, long duration of leaf wetness and cool weather (60-74 degrees F). Damage from downy mildew rarely results in the death of the plant.

Fusarium – is predominated by two forms, Fusarium solani a fungus often referred to as “sudden death syndrome” and Fusarium spp. which causes “dry rot”. F. solani causes yellow interveinal blotches, which eventually become necrotic, leaving green tissue along the leaf veins. Leafs often drop leaving petioles attaches to the stem. It is a weak pathogen requiring some kind of predisposing factor that stresses or injures the plant before it can become established, thus it is often a secondary invader. In tubers it causes brown rings and most importantly results in tuber rot during storage. Fusarium spp. produces light to dark lesions on tubers. The rotted tissue is relatively firm, but as the lesion matures, the tissues become dry and punky, and may contain areas that are yellow, pink or orange.

Black Spot (Diplocarpon rosae) – commonly appears on roses as black purplish spots and areas of yellowing (chlorosis) on upper surfaces of the leaf and on stems and branches. At the beginning of the growing season, blackspot will start to develop on the lower leaves and will move upward through the plant as the season progresses. Infected foliage eventually turns yellow and falls off the plant. Heavy infections can seriously defoliate a plant. Blackspot development is favored by warm (75°F), wet weather. Since the spores must have free water in order to germinate, avoid wetting the plants when irrigating (or do so early in the day, allowing plants sufficient time to dry).

Anthracnose – The symptoms of Anthracnose vary somewhat on different hosts, but typically the spots start on the leaves as water soaked areas and expand into brown spots which are roughly circular, reaching about 1/8 to 1/2 inch in diameter. The centers of these spots may drop out, producing a “shot hole” effect. Small, growing leaves may be distorted and severe spotting may cause entire leaves to blight. The fungal spores depend upon water for spread and infection; warm and humid rainy weather at frequent intervals is necessary for disease development. The spotted or striped cucumber beetle is another common vector that can carry the spores from plant to plant.

Sclerotinia – two species of Sclerotinia can cause disease on dahlias. Sclerotinia minor only infects stems or leaves in close contact with the soil. Once infection takes place, water-soaked brown necrotic areas develop. Plants eventually wilt and collapse. Upon examination of diseased plants one finds cottony fungal growth and numerous small, black, hard resting bodies called sclerotia (large reproductive structures) formed on the outside and inside of the stems. Sclerotinia sclerotiorum (white mold) can also infect lower leaves and stems, causing similar symptoms as S. minor, but S sclerotiorum also has an aerial spore that can infect upper leaves and flowers. S sclerotiorum forms larger sclerotia than those of S. minor. S sclerotiorum prefers cool and moist conditions and the aerial spores usually only infect injured leaves and flowers. The sclerotia may survive for years in infected soil.

Thielaviopsis (basicola)(black root rot) – is a soil inhabiting fungus that causes root rot and branch dieback on a number of woody and herbaceous plants. Soil temperatures between 55 and 65 degrees F favor the growth of Thielaviopsis, particularly if the soil is wet. Symptoms include stunting of foliage and root systems. Most dominant are blackened areas on roots. Often there is yellowing of leaves between the veins or along the margins (frequently mistaken for virus). Lower stems have sunken lesions.

Botrytis (cinerea) – commonly called Botrytis blight or gray mold first appears as a fuzzy white growth on the plant, but quickly darkens to a gray color. Spores multiply rapidly and can thrive on many different sources of nutrients. Botrytis is a weak pathogen that must have a food source before it can invade the plant, such as nutrients leaking from wounded plant parts or dying tissues such as old flower petals or leaves. Botrytis thrives in conditions of high humidity, poor air circulation and cool temperature. As with many other fungi, Botrytis has been known to develop resistance to many fungicides, particularly systemic fungicides when used exclusively over a period of time.

Fungus Gnats – while not a fungus, these small (1/16 in) insects feed off fungus and decaying plant material. They are slender, mosquito-like insects with long legs and many-segmented thread-like antennae. The gnats are often seen swarming around plants when fungus is present. While little threat to mature plants, they can damage young seedlings by feeding on roots and tunneling into stems, causing plant death. In addition, the larvae and adult fly are capable of spreading fungal diseases including Pythium, Thielaviopsis, and Botrytis. They can be treated with a general purpose insecticide such as Orthene, Diazinon, Talstar or Gnatrol.

In summary, a complete fungicide program would begin with a treatment with a fungicide (e.g. Banrot, Banol, Truban, Mycostop) effective against “water mold” and soil borne fungi for cuttings, seedlings or tubers started in soil or a soil-less medium. When planting out, a soil drench with a fungicide (e.g. Aliette, Terraclor, Terraguard, Subdue) taken up through the roots will prevent root and stem rots, During the growing season a foliar fungicide spray (e.g. Benefit, Cleary 3336, Consyst, Rubigan) effective against powdery mildew and/or blackspot should be used. Finally, before storing tubers or corms, a spray or dip with a broad spectrum contact fungicide (e.g. Chipco 26019, Daconil, Ditrane, Ornalin) will help protect against rot during storage.

Because of the different formulations, concentrations and labeling amongst fungicides using the same chemical from the same or different manufacturers, there may be some inaccuracies in simplifying pathogen vs. fungicide in a table format. Manufacturer labels, Agricultural Chemicals by W.T. Thomson (Book IV – Fungicides), Penn State University Cooperative Extension and other sources were used as references. Consult your County Agricultural Agent for specific use of a pesticide. Where trade names are used no endorsement or discrimination is intended by the author.

 – Bob Romano