In this issue

§      New insecticide products for the greenhouse
§      Myrothecium leaf spot on New Guinea impatiens
§      Control of anthracnose on ornamental lupines with fungicides
§      Screening for resistance in oomycetes to Subdue and assessing alternatives
§      Greenhouse insect management discussions – February 13

New insecticide products for the greenhouse

Dave Smitley
Entomology

Several new products have come out in the last two years that will help greenhouse growers keep pests under control. I am organizing them by pest, so you can see how they are best used.

Spider mites

Several new miticides are now available. They are safe and effective. Floramite is probably the most consistent and effective of the group in the greenhouse, with Hexygon DF close behind. These two and Ovation are all better than the old standard, Avid. Tetrasan is also a good miticide, but did not last quite as long in our 2002 research test.

Thrips

Only Pedestal is new this year, and it does not provide the same level of control that we get from Conserve SC and Mesurol. Conserve has become the standard, although Avid still works well most of the time.

Aphids

Endeavor gives us a low-toxicity option to Orthene 97 and Thiodan for aphid control. Aphids stop feeding within a few hours of application, but may remain alive for several days. Marathon II, applied as foliar spray, tends to be less active than Marathon applied as a granular or soil drench. It works on aphids, but we need some more testing to see how it compares with other products.

Whitefly

Endeavor also works on whitefly. Marathon as a soil drench is still the standard for whitefly control, but Endeavor provides another option to Sanmite, Orthene 97, Distance and Azatin as foliar sprays. Marathon II does not have whitefly on the label for foliar sprays because it doesn’t work well that way.

New products in development

Gowan is working on a new abamectin-type insecticide-miticide that works well for spider mites. This product would be similar to Avid, but hopefully will arrive without any resistance problems. Three new nicotinyl compounds, very similar to Marathon, are being developed. The first one, thiamethoxam, is being developed by Syngenta, and will be called Flagship. Syngenta is hoping for a label sometime in 2003. This will be a welcome alternative to Marathon and should have many of the same properties. FMC and Valent are also developing nicotinyl insecticides. They are numbered compounds at this time.

Myrothecium leaf spot on New Guinea impatiens

Jan Byrne
Diagnostic Services
Michigan State University

The diagnostic lab has seen several cases of Myrothecium on New Guinea impatiens in the last few weeks. The symptoms of this fungal pathogen are slightly sunken, circular leaf spots that often contain a dark ring. I have noticed many (but not all) of the leaf spots began at the leaf margin and moved inward. Young plants with succulent tissue are particularly susceptible. Myrothecium also commonly infects wounded tissue and plants under stress. In addition to New Guinea impatiens, this pathogen causes leaf spots and shot holes on pansy, begonia, gardenia, and gloxinia.

Under suitable environmental conditions Myrothecium forms small black dots within the infected tissue. These dots, or sporodochia, are specialized spore-producing structures and are a good diagnostic feature to look for.

Cultural factors that help control this disease include minimizing leaf wetness, avoiding extremely succulent growth, and careful handling of the plants to reduce wounding. Fungicide applications will probably also be necessary. Currently, there is not a great deal of information to draw from when it comes to making fungicide control recommendations on New Guinea impatiens. Based on information taken from control on pansies, I recommend applications of chlorothalonil to control foliar infections. Applications will need to be repeated in situations of severe disease pressure.

Please feel free to contact me (517-355-3504) if you have questions regarding diagnosis or control of this disease.

Control of anthracnose on ornamental lupines with fungicides

Willie Kirk
Plant Pathology

Lupines (Lupinus spp.) are popularly grown as perennial ornamentals in North America. Anthracnose (Colletotrichum gloeosporioides) was observed from about 1997 in Russell hybrid lupines in several nurseries in Michigan. Anthracnose of lupine is seed-borne and occurs in most regions where forage lupines are grown. Seedlings are very vulnerable and show wilting, leaf and stem necrosis leading to death. Growers in Michigan have noted that fungicides registered for use on lupines were not effective for control of anthracnose, especially when the disease occurred early in the season. The objectives of this study were to evaluate whether any fungicides known to be effective against other anthracnose diseases on legumes could effectively control anthracnose in lupine.

Materials and methods

Efficacy of fungicides against early symptoms of seed-borne foliar anthracnose was evaluated. Foliar fungicide applications were applied once 24 hours after inoculation with a backpack-mounted R&D spray boom delivering 40 gal/A (40 p.s.i). A single evaluation was made 35 days after planting, 15 days after inoculation.

Results

Evaluation of foliar fungicide treatments for seedling lupine anthracnose control

The untreated non-inoculated control had an index of 20.2, which was significantly lower than the all other treatments (Table 1). The inoculated control reached an index of 100 untreated controls 15 days after inoculation. Several treatments had significantly lower indices than the untreated control. The most effective products with indices between 40 and 50 were Cygnus 50WP 0.4 lb/A and Daconil WS 6SC 1.5 pt/A.

Table 1. Efficacy of foliar fungicides and plant defense system stimulators against seed-borne anthracnose of lupines.

¹ An anthracnose index was calculated by counting the number of plants from each sample of 50 plants falling onto class 0 = no visible symptoms; 1 = 1 - 2% of plant tissue with lesions; 2 = 3 - 10% of plant tissue with lesions; 3 = 11 - 50% of plant tissue with lesions; 4 = 51 - 100% of plant tissue defoliation. The number of plants in each class is multiplied by the class number and summed. The sum is multiplied by a constant to express as a percentage. Indices of 0 - 25 cover the range 0 - 2%; 26 - 50 (QE; quite effective) cover the range 3 - 10%; 50 – 75 (ME; moderately effective) cover the range 11 - 50% foliar symptoms and > 75 (NE; not effective) cover the range 51 - 100% foliar symptoms and plant death.

² Values followed by the same letter are not significantly different at P = 0.05 (Tukey Multiple Comparison).

Screening for resistance in oomycetes to Subdue and assessing alternatives

Willie Kirk, Plant Pathology

Oomycetes, including Pythium, Perenospara and Phytophthora species cause wilt, downy mildew and folair blight, respectively in perennials. There have been reports of failure to control these diseases in perennials, and we suspect some oomycetes may have become resistant to the active ingredient in Subdue products (mefenoxam and metalaxyl).

It is in the interest of the industry to determine if target pathogens are resistant to Subdue products and to evaluate alternatives if resistant isolates of these pathogens exist. We are seeking samples of perennial plants (preferably in flats) in which loss of control of oomycetes has been observed. We already have samples of downy mildew in Lamium and Rudbeckia, and we are looking for examples with Pythium and Phytophthora. If you have any suitable samples, please contact Willie Kirk and samples can be collected.

I can be reached at: 517-353-4481, 517-355-4754 or kirkw@msu.edu.

Greenhouse insect management discussions
February 13, West Central Michigan

Tom Dudek
District Horticulture Agent

Strategies for controlling thirps, aphids, spider mites and other insects in 2003.

Dr. David Smitley, Entomology Department, and I will be conducting informal round table discussions on Thursday, February 13, 2002 at the following locations. Please plan to join us at the site nearest to you.

9:00 a.m., Sawyer Nursery, 5401 Port Sheldon Road, Hudsonville
11:00 a.m., Hilltop Gardens Greenhouse, 7516 Buchanan, Allendale
1:30 p.m., Willbrant Farms Greenhouse, 700 Witham Road, North Muskegon