Diseases of Annual Vinca in the Greenhouse and Landscape

Leaf Spot Diseases

Symptoms of both diseases first appear on the lower leaves as small brown to black spots (Figure 1). Spots on the leaves, shoots, and petioles will range from the size of a pinhead up to ¹/8 inch in diameter. Alternating light and dark rings of dead tissue give the larger spots a target spot or bull's-eye appearance. As more spots develop, the diseased leaves turn yellow and eventually fall from the plant. Typically, leaf drop starts at the base of the shoot and continues upward until all but the youngest leaves are lost.

Cultivars of annual vinca differ in their sensitivity to Alternaria leaf spot. The susceptibility of some popular annual vinca cultivars to Alternaria leaf spot is listed in Table 1.

Table 1. Susceptibility Of Cultivars Of Annual Vinca To Alternaria Leaf Spot.

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Low	Medium	High
Tropicana Rose	Tropicana Blush	Tropicana Pink
Tropicana Bright Eye	Parasol	Cooler Grape
	Little Blanche	Cooler Peppermint
		Cooler Blush

 Table 2. Fungicides Recommended For The Control Of Alternaria And Ulocladium Leaf Spot.

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Fungicide	Rate Per 100 Gal.	Comments
Chipco 26019 50W	1-2 lb.	Apply as needed at 7- to 14-day intervals.
Chipco 26GT	1-2.5 qt.	Apply as needed at 7- to 14-day intervals
Heritage 50W	1-4 oz.	Apply as needed at 14- to 28-day intervals

Phytophthora Blight

Typical symptoms of Phytophthora blight on vinca can include the sudden flagging of one shoot (Figure 2) or the wilting of an entire plant. Graygreen, water-soaked lesions are usually found at the base of the wilted shoots. Later, sunken, reddish brown girdling cankers may be found along the entire main stem at the base of blighted lateral shoots (Figure 3 and Figure 4). A root rot may be seen in landscape beds infested with the causal fungus P. parasitica. Plant death may occur within 1 to 2 weeks of the first appearance of symptoms.

In production greenhouses, strict sanitation is the key to controlling Phytophthora blight. Clear benches or floors of debris from the previous crop. Clean propagation and production areas, transplanting equipment, and recycled pots and flats with a commercial disinfectant. Use fresh potting media and minimize water splash onto the foliage when irrigating plant material. Inspect each vinca crop weekly and discard any flats or pots containing diseased plants. To prevent disease outbreaks in production greenhouses, apply the fungicide Aliette WDG as a heavy spray/drench at the rate and interval specified in Table 3. Normally, two to three treatments will be required before the finished crop is ready to ship.

Table 3. Fungicides Recommended for the Control of Phytophthora Blight.

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Fungicide	Rate Per 100 Gal.	Comments
Aliette WDG	1.25-4 lb.	Spray to wet foliage. Apply monthly. Repeated applications of the high rate of Aliette may damage vinca.

To avoid accidentally introducing Phytophthora blight into landscape beds, do not buy irregular, discolored, or otherwise poor quality plant material. Uniform plant height and foliage color across all flats or pots on display are usually good indicators of quality bedding plants.

Avoid installing vinca in the same landscape beds year after year. Alternate growing vinca with other summer annual or perennial flowers. Delay the planting of vinca until the soil warms in the spring to 70°F. Vinca prefer a well-drained sandy or sandy loam soil. In poorly drained clay soils, plant on a raised bed in soil amended with aged pine or hardwood bark. Avoid overfertilizing vinca, particularly with a fertilizer that contains high concentrations of the ammonium form of nitrogen or urea. For best results, apply a slow-release fertilizer or calcium nitrate. To avoid wetting the leaves, water vinca with a drip irrigation system or soaker hose. When using overhead sprinklers, water beds between 2 a.m. and sunrise or at midday. Immediately remove and discard diseased plants.

Generally, avoid installation of vinca in Phytophthora-infested landscape beds. Although all popular vinca cultivars are susceptible to Phytophthora blight, the cultivars ‘Little Bright Eye’ and ‘Little Pinkie’ may suffer less damage. Not all annuals and perennials are susceptible to attack by the causal fungus of Phytophthora blight. Possible replacements for vinca in Phytophthora-infested beds include ageratum, begonia, celosia, coneflower, geranium, marigold, scabosia, thyme, verbena, and zinnia.

Fumigation with Vapam or Basamid or soil solarization will protect vinca for one season from hytophthora blight. See Extension publications ANR-0030, “Nematode Control in the Home Garden,” and ANR-0713, “Soil Solarization for the Control of Nematodes and Soil-Borne Diseases,” for additional information concerning soil fumigation and solarization, respectively. Vapam and Basamid are RESTRICTED USE PESTICIDES that require Pesticide Applicator Certification for their purchase and use.

Fungicides will give some protection from Phytophthora blight. For effective control, apply a fungicide (see Table 3) at monthly intervals starting immediately after plants are set out in the spring through early fall.

Rhizoctonia Crown Rot and Web Blight

Vinca grown in flats or pots that are damaged by Rhizoctonia crown rot often topple over or snap-off at or just above the soil line (this is post-emergence damping-off). Girdling cankers that form at the soil line will cause a sudden wilting and death of vinca. At times, the thread-like hyphae R. solani may be seen suspended between the stems and lowest leaves.

Web blight first appears as semicircular, water-soaked lesions at the leaf-petiole junction on leaves at or just above the soil line. Infected leaves are quickly blighted and killed. Mats of dried tan-colored dead leaves cling to the green shoots (Figure 5). Left unchecked, all but the youngest leaves will be blighted and eventually the diseased plants die. Following a heavy dew, the thread-like hyphae of R. solani can be seen on and suspended between symptomatic leaves. Symptoms of the more common and damaging Phytophthora blight are easily confused with those of web blight.

Cultural practices recommended for control of Rhizoctonia crown rot on vinca in production greenhouses are the same as those previously described for Phytophthora blight. Fungicides, when applied as a heavy spray/drench at specified intervals through the production cycle, will control Rhizoctonia crown rot. Since web blight is rarely a problem in landscape plantings, preventive fungicide sprays are usually unnecessary. Should damage appear, apply a recommended fungicide (see Table 4). Should weather conditions favor the disease, make several additional applications at 7- to 10-day intervals.

Table 4. Fungicides Recommended For The Control Of Rhizoctonia Crown Rot And Web Blight.

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Fungicide	Rate Per 100 Gal.	Comments
Chipco 26019 50W	1-2 lb.	Apply every 7 to 10 days when conditions favor disease.
Chipco 26GT	1-2.5 qt.
Cleary's 3336 50W	12-16 oz.
Cleary's 3336 4.5F	20 fl. oz.
OPH 6672 4.5F	10-20 fl. oz.
OPH 6672 50W	12-16 oz.
Contrast 70 WSP	3-6 oz.	Repeat every 21 to 28 days as needed

Botrytis Blight

The causal fungus of Botrytis blight, Botrytis cinerea, usually invades only wounded or senescent plant tissue, particularly flower petals, as well as the leaf debris of dead plants. Small, water-soaked spots on the flower petals, which are usually the first symptoms seen, quickly expand from tan to gray-brown blotches (Figure 6). Fast-spreading, circular spots, often with alternating light and dark rings or bull’s-eye pattern, will appear on leaves where blighted petals or other plant debris have fallen. Girdling gray-green, water-soaked cankers, which are similar to those associated with Phytophthora blight, may also be seen. Blighted tissues are often covered with the fuzzy, graybrown hyphae and spore clusters of the causal fungus.

In a production greenhouse, good sanitation is the key to the control of Botrytis blight on bedding and floral crops. In a dirty greenhouse, even the best fungicides available will often fail to give the desired level of control. Sanitation and cultural practices recommended for control of Botrytis blight in production greenhouses are detailed in Extension publication ANR-0753, “Identification and Control of Botrytis Blight on Floral Crops and Woody Ornamentals.”

Fungicides may be applied to vinca as a foliar spray or smoke fumigant (greenhouse only). The smoke fumigant formulation of chlorothalonil must be used before the blooms open. Shortening the interval between fungicide applications in late winter and early spring during lengthy periods of cloudy, cool, and wet weather is often necessary for effective disease control. Apply a recommended fungicide (see Table 5) just before shipping finished plants. Fungicides are rarely needed to control Botrytis blight on vinca in landscape beds. See Extension publication ANR-500-B, Alabama Pest Management Handbook—Volume 2, for additional information.

Table 5. Fungicides Recommended For The Control Of Botrytis Blight.

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Fungicide	Rate Per 100 Gal.	Comments
Chipco 26019 50W	1-2 lb.	Apply every 7 to 10 days when conditions favor disease. Use higher rate at shorter interval when disease is active.
Chipco 26GT	1-2.5 qt.
Cleary's 3336 50W	12-16 oz.
Cleary's 3336 4.5F	16-20 fl. oz.
Cygnus	1.6-3.2 oz.
OPH 6672 50W	12-16 oz.
OPH 6672 4.5F	16-20 fl oz.

Black Root Rot

Poor growth, yellow foliage, and low plant vigor are symptoms common to all root rot diseases. These symptoms can easily be confused with those of a nutritional deficiency or toxicity as well as other root rot diseases. At early stages of this disease, black or brown bands develop along the normally white roots. With a good hand lens, the bullet-shaped spores of the causal fungus can be seen within the banded areas. As the disease progresses, the colonized roots turn black and mushy (Figure 7).

In greenhouses, good sanitation is a critical component of a disease management program. Routinely clean and disinfect all work surfaces and benches with a 5% solution of chlorine beach or a commercial disinfectant. Additional sanitation practices are described under Phytophthora blight. The causal fungus is an inhabitant of peat bogs and is often introduced in peat-based soilless potting media. To minimize disease severity, follow recommended fertilization practices for vinca production. Also, avoid overwatering or under watering vinca.

Fungicides, when applied as a heavy spray/drench at recommended rates and treatment intervals (see Table 6), will control black root rot. Preventive fungicide drenches are recommended for the control of black root rot on greenhouse crops but not in landscape beds.

Table 6. Fungicides Recommended For The Control Of Black Root Rot.

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Fungicide	Rate Per 100 Gal.	Comments
Cleary's 3336 50W	12-16 oz.	Apply as a preventive soil drench every 14 days until finished plants are sold. See labels for additional application instructions.
OPH 6672 50W	12-16 fl. oz.
Cleary's 3336 4.5F	16-20 fl. oz.
OPH 6672 4.5F	16-20 fl. oz.
Banset 40W	4-8 oz.
Terraguard 50W	2-4 oz.	Apply drench every 21 to 28 days as needed.

Tomato Spotted Wilt

TSWV has a wide host range, and symptoms can be quite variable. On vinca, typical symptoms include yellowing, stunting, and distortion (‘possum ear’) of young leaves along with small, black concentric rings, spots, or line patterns on those leaves (Figure 8). Flowers on infected vinca are discolored and malformed. Diseased plants are usually severely stunted.

Before you purchase any plants, inspect vinca and other bedding plants for symptoms of TSWV. Avoid planting vinca and other TSWV-susceptible summer annuals in beds adjacent to vegetable gardens because many broadleaf vegetables are also susceptible to TSWV. Remove diseased plants as symptoms appear. Insecticides applied to control the thrips vector are ineffective in slowing the spread of TSWV.