Greenhouse Update

Columbine Powdery Mildew

Columbine powdery mildew caused by the fungus Erysiphe aquilegiae was observed on potted columbines (Aquilegia canadensis) in the greenhouse. Most of the leaflets on the plants were covered in white powdery patches. The powdery mildew fungus thrives in high humidity conditions (greater than 90%) and dry leaf surfaces. The ideal temperature for infection is 68 - 80ºF. The white powdery patches may cover the entire leaflets and sometimes the leaves may turn purplish.

Spacing plants and increasing air circulation can help to control powdery mildew. Maintain low humidity conditions with increased air circulation and avoid overfertilizing with nitrogen. Some of the fungicides that are effective for powdery mildew include Terraguard (FRAC group 3), Eagle (3), Postiva (3+7), Broadform (7+11), Mural (11+7), and Palladium (12+9).

Fungicides do best when applied early before symptoms develop. Use the shorter application interval when the environmental conditions favor disease and rotate between fungicides in different FRAC groups. Read and follow the directions on the label.

For more information check out the New York and New England Management Guidelines for Greenhouse Floriculture and Herbaceous Ornamentals. (https://www.negreenhouse.org/pest-guides.html

Botrytis Blight on New Guinea Impatiens

Botrytis leaf spots were observed on New Guinea impatiens growing under hanging baskets. The spots were observed on the leaves where flower petals from plants in the hanging baskets above fall on the plants below.

Botrytis is caused by the fungus Botrytis cinerea and it isone of the most common diseases in the greenhouse. Most greenhouse plants are susceptible to botrytis and infection can occur in all stages of production. Botrytis infection is dependent on a moisture film on the surfaces of leaves and flowers such as that caused by condensation. Botrytis is therefore an issue when high humidity (above 90%) and decaying tissue are present. Botrytis blight is likely to attack senescing flower petals, so plants growing underneath lines of flowering hanging baskets are most at risk. A moist and humid environment in the greenhouse is ideal for the pathogen’s sporulation. Botrytis can cause leaf and flower spots and can also cause stem cankers. It produces gray fuzzy spores on the surface of the infected tissue.

To manage Botrytis, remove all dead or dying plants or plant parts (particularly flowers), and lower humidity levels to below 90%. Water early in the day to prevent prolonged periods of leaf wetness. Increase plant spacing for good air circulation and use HAF fans to increase air circulation in the greenhouse. Do not place plants on benches directly beneath hanging baskets and avoid water splash on foliage during watering.

Some of the most effective fungicides for Botrytis include Broadform (FRAC groups 7+11), Daconil (M5), Decree (7), Affirm WDG (19), Mural (11+7), and Palladium (9+12). Read and follow the directions on the label.

For more information check out the New York and New England Management Guidelines for Greenhouse Floriculture and Herbaceous Ornamentals. (https://www.negreenhouse.org/pest-guides.html

Two spotted spider mites on Thunbergia

Two spotted spider mites were observed on Black-eyed Susan vine (Thunbergia alata). Thunbergias are very susceptible to spider mites. Two-spotted spider mites (Tetranychus urticae) are one of the major pests of many greenhouse crops. They feed primarily on the undersides of leaves of a wide range of ornamental and vegetable crops. They pierce cells with their stylet-like mouthparts and suck out the contents. The feeding injury results in faint chlorosis or stippling on upper leaf surfaces. Whole leaves turn yellow and may cause leaf drop and death of the plant. In severe infestations, plants may be covered with webbing. Water-stressed plants are more susceptible to spider mite damage. Look on the undersides of the leaves for the mites and their eggs.

When plants are heavily infested, mites fall to the ground and walk to other plants. They can move from one leaf to another if plants are touching each other, and also use the silk threads of their webs to move to other leaves or plants. They can also be moved by air currents in the greenhouse.

To prevent two spotted spider mites (TSSM), avoid over-fertilizing plants. Increased fertility levels encourage lush, succulent growth, which is ideal for TSSM. Eliminate weeds in and around greenhouses that can harbor spider mites

Biological management: The most effective biological control agent for two spotted spider mites is the predatory mite Phytoseiulus persimilis. P. persimilis feeds only on TSSM and will prey upon all life stages of this pest, but they do have a preference for eggs. P. persimilis develops faster than TSSM when temperatures are 59-68°F. Above 68°F the average generation time of P. persimilis is shorter than that of TSSM, so they are less effective under warmer conditions.

The predatory mite Neoseiulus (Amblyseius) californicus is also effective for TSSM and is more tolerant of higher temperatures and lower humidity. In areas where the TSSM population is high (hotspots), the pest can be controlled by the gall midge Feltiella acarisuga. The larvae of Feltiella feed on all stages of TSSM.

Chemical management: For chemical management of TSSM, contact or translaminar miticides can be used. Translaminar miticides are more effective because they penetrate leaf tissues and form a reservoir of active ingredient within the leaf, which provides extended residual activity. Successful treatment requires good coverage on the undersides of the leaves, and repeat applications may be needed during warm weather. Miticides with translaminar activity include Avid (IRAC group 6), Sirocco (6+20), Pylon (13). Contact miticides include Akari (21A), Floramite SC (20) and Sultan (25). Always read and follow label instructions carefully.

For more information check out the New York and New England Management Guidelines for Greenhouse Floriculture and Herbaceous Ornamentals. (https://www.negreenhouse.org/pest-guides.html

- Geoffrey Njue, Extension Specialist, UMass Extension Greenhouse Crops and Floriculture Program, with Angela Madeiras, Extension Educator and Diagnostician, UMass Extension Plant Diagnostic Lab