Don’t Let Leaf Spot Spoil Your Vincas

Linda Benedict, Chen, Yan, Owings, Allen D.  |  3/5/2007 11:03:34 PM

Yan Chen and Allen Owings

Annual vinca, also referred to as periwinkle by many home gardeners and industry professionals, is one of the best-selling bedding plants in the Southeastern United States. It is adaptive to summer heat, sand and silt-based soils, acid soils and low rainfall. Once established in the landscape, vinca provides season-long color with nonstop bloom performance. A wide variety of colors and sizes is available for landscape use.

The vinca is not without its problems, however. During the spring months, vinca is susceptible to root and stem rot. Another major concern is a disease called Alternaria leaf spot. These leaf spots are prevalent in the late summer and fall months and can sometimes be observed in new plantings. Individual Alternaria leaf spots are tan to black, pinhead to penny-sized lesions. The spots develop on the leaves starting at bottom of the stems and progressing to the top. Infected leaves will turn yellow and fall off. A severe infection can strip the stems and render the planting unattractive. Heat and humidity in late summer provide ideal conditions for leaf spot infection. Effective control requires frequent fungicide applications that are expensive and may pose negative environmental impact. Other factors that may affect disease incidence include planting techniques and variety selections.

A field study conducted in 2005 and repeated in 2006 at the LSU AgCenter’s Hammond Research Station investigated how these factors would affect the development of Alternaria leaf spots. Four-inch pots of 6-week-old vinca plants were planted on April 1 and May 1 in a landscape bed mulched with pine bark and irrigated. Disease severity was assessed in June, July and August.

Disease-resistant Varieties
Vinca varieties can be categorized as open-pollinated upright, F1 hybrid upright, open-pollinated trailing, and cuttingpropagated. In the Hammond study, the scientists compared varieties Cooler Hot Rose (open-pollinated), Titan Rose (F1) and trailing vinca Mediterranean Lilac (open-pollinated) with Nirvana Rose and Nirvana Red, both of which are cuttingpropagated.

The open-pollinated Cooler Hot Rose had the least leaf spot infection. Both Nirvana varieties were as susceptible as the hybrid Titan Rose. The trailing Mediterranean Lilac was the most susceptible of all varieties grown. Because leaf spots are transmitted by raindrop splashes, trailing varieties are infected more easily than upright varieties. Based on these results, the classic open-pollinated vinca varieties are relatively resistant to Alternaria leaf spots.

Planting Late
As a warm-season plant, vinca has a wide window for planting and can be planted from April to June. Many landscapers like to plant in early April to get the color effect as soon as possible in the spring. Results indicated that planting late (May 1) significantly reduced the amount of Alternaria leaf spots over an April 1 planting date. Plants planted in May were still acceptable in August, while the plants planted in April needed to be removed. Because both planting times gave about 14 weeks of blooms, landscape professionals can choose planting date to meet their specific time frame for color display.

Fertilize Wisely
Though extremely adaptive to poor soil in their native habitat, newer vinca varieties will show nutrient deficiency symptoms when left unfertilized after planting. In the Hammond study, the trailing vinca Mediterranean Lilac performed better with less nitrogen than the other varieties. A complete formula controlled-release fertilizer such as 13-13-13 (N-P-K) at 0.5 pound nitrogen per 1,000 square feet is sufficient for trailing varieties for most soils. For upright varieties, the recommendation is 1 pound nitrogen per 1,000 square feet. Higher fertilizer rates (4 pounds of nitrogen per 1,000 square feet) did not improve plant quality but encouraged the development of disease with excessive lush growth.

We also evaluated the effects of Aliette T&O fungicide as a preventive spray applied 1) at planting, 2) at planting and after leaf spots were first observed, or 3) every four weeks over the season. Applying Aliette fungicide every four weeks only gave a slight improvement in plant quality, while applying it at planting or at disease observation did not make any difference in how much leaf spot occurred.

Based on these findings, leaf spot lesions can be reduced with the use of less susceptible open-pollinated and upright varieties combined with the proper level of fertilizer. A fertilizer rate of 1 pound nitrogen per 1,000 square feet should be sufficient for vinca planted in most landscape soils. Planting later in the spring (May 1) will delay disease development and delay the need to replace the planting into August.

(This article was published in the winter 2007 issue of Louisiana Agriculture.)
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