Fall Pansies: Fighting Black Root Rot and Phytophthora

Few crops can compete with the performance of pansies and violas during the fall season. For many regions, fall pansies are a landscape staple, providing impressive color through the autumn and winter months. In northern regions, pansies provide fresh fall colors and can overwinter to provide an early burst of color in the spring. Plant breeders have ensured there’s a lot to enjoy: a myriad of colors, flower sizes, and new plant habits. From the grower perspective, pansies also provide a welcome profit center that’s independent of spring.

Even with improved genetics, pansies are still a cool-season crop. Fall-flowering crops are typically started when the weather is quite warm, even hot, and therein lies the challenge from a disease-prevention standpoint. Two diseases of the root and crown area earn mentions for being particularly challenging for fall pansy production: Thielaviopsis and Phytophthora.

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Black Root Rot: Thielaviopsis

Thielaviopsis, also known as Black Root Rot, is a wasting disease of pansies that’s inhibited to some extent at a soil pH below 5.8. Warning signs include roots that darken and are reluctant to grow out of the original plug. Infestations are often random in appearance, with stunted pansies right alongside plants that are thriving. Unless you have a microscope and can identify Thielaviopsis spores in the roots, suspicious plants should be sent to a plant diagnostic lab for positive identification.

Plants infected with Thielaviopsis cannot be salvaged, meaning fungicide treatments can only be used to protect adjacent healthy plants. Given the susceptibility of pansies and the environmental stresses early in production, GGSPro recommends a preventative fungicide drench soon after transplanting. The products we recommend for Thielaviopsis also control Rhizoctonia.

Phytophthora

Phytophthora is seldom found in winter/spring-grown pansies, but can cause substantial losses in late summer and early fall. Warm temperatures and abundant splashing water favor this fast-moving algae. The Phytophthora zoospores (swimming stage) can readily move about and enter healthy plants via the drain holes in pots and flats. For this reason, growing the crop up off the ground is very desirable. Benches are ideal; however, inverted flats or 1-inch PVC pipes under the flats will serve the purpose.

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Effective fungicides may be applied to address Phytophthora, but to be successful, it’s important to reduce the risk for spread. Learning to identify the early symptoms as well as the conditions that make disease expression more likely is also key. Watch for sudden stem constriction and collapse at the crown. Rootshield Plus is a fungal-based preventative fungicide that includes Phytophthora prevention due to the addition of a second strain of Trichoderma to the original formulation. It’s also labeled for the prevention of Thielaviopsis.

Pansy Care: Additional Things to Look For

Pythium is another water mold that can cause root rot in fall pansies. Over-fertilization, poor drainage that leads to containers in standing water after heavy rains and irrigations, and heat stress can predispose fall pansies to this common foe.

Foliar diseases become more prevalent as the crop progresses and the weather cools. Anthracnose (Colletotrichum), Botrytis, and Cercospora are among the troublemakers.

Anthracnose

Anthracnose symptoms on pansies generally appear as large necrotic and chlorotic lesions that are somewhat irregularly shaped. Often, faint concentric rings can be detected within the necrotic tissue. Botrytis can be especially problematic as early blooms begin to senesce and “melt,” sometimes involving adjacent foliage. Cercospora often manifests itself with dark-purple spots, some of which have tan centers, giving rise to the fish-eye description.

Read and follow all label directions. The label is the law! Products other than those mentioned may also be safe and effective. Some pesticides may be restricted-use or unregistered in certain states. Few crops can compete with the performance of pansies and violas during the fall season. For many regions, fall pansies are a landscape staple, providing impressive color through the autumn and winter months. In northern regions, pansies provide fresh fall colors and can overwinter to provide an early burst of color in the spring. Plant breeders have ensured there’s a lot to enjoy: a myriad of colors, flower sizes, and new plant habits. From the grower perspective, pansies also provide a welcome profit center that’s independent of spring.

Even with improved genetics, pansies are still a cool-season crop. Fall-flowering crops are typically started when the weather is quite warm, even hot, and therein lies the challenge from a disease-prevention standpoint. Two diseases of the root and crown area earn mentions for being particularly challenging for fall pansy production: Thielaviopsis and Phytophthora.

Black Root Rot: Thielaviopsis

Thielaviopsis, also known as Black Root Rot, is a wasting disease of pansies that’s inhibited to some extent at a soil pH below 5.8. Warning signs include roots that darken and are reluctant to grow out of the original plug. Infestations are often random in appearance, with stunted pansies right alongside plants that are thriving. Unless you have a microscope and can identify Thielaviopsis spores in the roots, suspicious plants should be sent to a plant diagnostic lab for positive identification.

Plants infected with Thielaviopsis cannot be salvaged, meaning fungicide treatments can only be used to protect adjacent healthy plants. Given the susceptibility of pansies and the environmental stresses early in production, GGSPro recommends a preventative fungicide drench soon after transplanting. The products we recommend for Thielaviopsis also control

Phytophthora

Phytophthora is seldom found in winter/spring-grown pansies, but can cause substantial losses in late summer and early fall. Warm temperatures and abundant splashing water favor this fast-moving algae. The Phytophthora zoospores (swimming stage) can readily move about and enter healthy plants via the drain holes in pots and flats. For this reason, growing the crop up off the ground is very desirable. Benches are ideal; however, inverted flats or 1-inch PVC pipes under the flats will serve the purpose.

Effective fungicides may be applied to address Phytophthora, but to be successful, it’s important to reduce the risk for spread. Learning to identify the early symptoms as well as the conditions that make disease expression more likely is also key. Watch for sudden stem constriction and collapse at the crown. Rootshield Plus is a fungal-based preventative fungicide that includes Phytophthora prevention due to the addition of a second strain of Trichoderma to the original formulation. It’s also labeled for the prevention of Thielaviopsis.

Pansy Care: Additional Things to Look For

Pythium is another water mold that can cause root rot in fall pansies. Over-fertilization, poor drainage that leads to containers in standing water after heavy rains and irrigations, and heat stress can predispose fall pansies to this common foe.

Foliar diseases become more prevalent as the crop progresses and the weather cools. Anthracnose (Colletotrichum), Botrytis, and Cercospora are among the troublemakers.

Anthracnose

Anthracnose symptoms on pansies generally appear as large necrotic and chlorotic lesions that are somewhat irregularly shaped. Often, faint concentric rings can be detected within the necrotic tissue. Botrytis can be especially problematic as early blooms begin to senesce and “melt,” sometimes involving adjacent foliage. Cercospora often manifests itself with dark-purple spots, some of which have tan centers, giving rise to the fish-eye description.

Read and follow all label directions. The label is the law! Products other than those mentioned may also be safe and effective. Some pesticides may be restricted-use or unregistered in certain states.


Editor’s Note: More information on the diseases included in this article, as well as other diseases that affect pansies, is available through GGSPro and the GGSPro 5th Edition Reference Guide, which includes cultural and chemical controls, as well as helpful diagnostic pictures.

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