Greenhouse TPM/IPM ReportCentral Maryland Research and Education CenterEllicott City, Maryland

April 8, 2020From: Stanton Gill, Extension Specialist – IPM for Greenhouse and Nurseries, Central Maryland Research and Education Center, University of Maryland ExtensionKaren Rane, Plant Pathologist, Director of the Plant Diagnostic Clinic, University of Maryland ExtensionAndrew Ristvey, Extension Specialist, Wye Research and Education Center, University of Maryland ExtensionJohn Erwin, Professor and Chair, Department of Plant and Landscape Architecture, University of MarylandSuzanne Klick, Technician, CMREC, University of Maryland Extension

Greenhouse and Cut Flower UpdateBy: Stanton Gill

Again, thanks to all of you who have kept us updated on how your sales are doing this spring. First, the good news. The sales of herb plants and vegetable plants continue to be brisk this spring. Many home-bound folks are gearing up their gardens at home. It might be something that they can do with their children.

Flowers sales were staying pretty good for homeowner sales in March but dropped off for many in April. We had several local garden centers who decided to close down rather than risk that their employees become exposed to Covid-19. The garden centers that have remained open are reporting that their sales have shot up as much as 3 times normal sales in the last 2 weeks. People staying at home are bored and looking for something to do in their landscapes and are planting flowers.

On the other end of big sales, several of the big box stores started restricting the number of customers allowed into the store to maintain social distancing. This change is resulting in lines outside stores with 6 ft spacing between people. This has slowed sales of plants being sold at several of the big box stores. Several of our Maryland growers grow plants to sell into this market so it is influencing the sales in the last week.

Cut flowers and hanging baskets should become popular with Easter coming up, followed by Mother’s day. Let us hope the florists, local grocery stores, and local farm markets will increase orders for these two holidays. Cut flower and cut woody stem growers report that wedding flower sales are way down. Weddings are either being delayed or greatly reduced in size since large gatherings are discouraged. Hopefully, by summer this situation will change, but the date keeps being pushed back for when we are out of the woods. If you are a cut flower grower contact us if you are having any problems or if you found a better way to sell flowers in these tough times.

Greenhouse Insect UpdateBy: Stanton Gill

First off, thanks to all of the greenhouse operators who we have called this week to get an industry view of what is going on.

Aphids and thrips populations are staying down in most greenhouses over the last week which is great news. One reason is because several greenhouses had customers cut back on pansy sales, so a lot of pansy plants were thrown on the compost pile in the last 2 weeks. Generally, we would see green peach aphid and melon aphid on this crop.

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The really good news is the thrips population, based on several calls this week, are staying down at this time. These low levels are somewhat tied into the weather. Thrips thrive in much warmers and less rainy outdoor weather. If you did not bring in thrips on cuttings or seed plug plants, then the thrips population is generally down. We are not getting strong wind currents from the south that bring up a large quantity of thrips on the northern bound jet streams of air. Keep in mind as we progress in April and early May, we will increasingly see the jet streams from the south begin to predominate, bringing thrips up from the south which will migrate into greenhouses.

If you are considering biological control, which I highly recommend instead of relying solely on insecticide applications for your protection, it is still a good time to purchase predacious mites, Amblyseius cucumeris. You can get them out onto the crop using blowers or spread them by hand. For baskets and large containers, I would suggest using the sachets with the mite population mixed with grain mites. The beneficial mites move out of the sachet and establish on the foliage of the plant. The mites will feed on first instar thrips. We have used this method for many years in several Maryland greenhouses with good success if you start before a thrips population builds up.

If you started purple flash or red missile pepper plants as banker plants, you can move them into the greenhouse in late April to May and order Orius insidious, minute pirate bugs, to release later. The minute pirate bug will feed on the first and second instar stages of thrips as well as adult thrips.

If you are having insect problems, call me at 410-868-9400 or e-mail me at Sgill@umd.edu. All of us in the University of Maryland Extension are here to help you.

Melon aphids vary in color from yellowish to green to black; all color forms have black cornicles

Tap plants over white paper on a clipboard to monitor for thrips

Holding Bedding Plants for Delayed Sales During the CoVID OutbreakBy: John Erwin, Professor and Chair, Plant Science and Landscape Architecture

The Plant Science and Landscape Architecture Department at the University of Maryland – College Park (PSLA) has hired new faculty with greenhouse/nursery production expertise. This article is the beginning of a new focus on greenhouse ornamental and food production and nursery production in PSLA. In the coming months I, and other UMD-CP faculty, will be providing useful, timely articles to help Maryland greenhouse and nursery growers continue to be successful; you already have an excellent IPM network here so I/we will focus on other timely production issues such as nutrition, plant growth regulators (PGRs), lighting, media etc. Any article we write will be in collaboration with University of Maryland Extension staff that have a long history of working with you. If you have any questions, please feel free to call me (see below).

Greenhouse bedding plant and vegetable transplant producers are experiencing unprecedented pressure to hold product now for May - June sales because of delayed sales due to the CoVID outbreak. In many states, garden centers are closed (Michigan) or customer volume per store is being controlled to observe social distancing

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(Maryland). Growers should be hopeful there may be some easing of social distancing requirements in May. What do you do with product that is ready to sell now but need to hold it for another 4-6 weeks? You really only have 2 options: 1) cut product back and sell 4-6 weeks later, or 2) apply a growth retardant to reduce stem elongation now. Below are the most common plant growth retardant treatments used to hold product for approximately 3 weeks.

PGR Drenches:By far the most common PGR drench application to hold product is 2 ppm of Bonzi (4 ppm if hot as in May/June). Advantages of Bonzi drenches include bloom size is not reduced and flowering is not delayed. Disadvantages include increased labor to apply the drench compared to sprays (unless you have a drip tube system) and you cannot apply this drench to some key crops; namely begonias, vinca, New Guinea impatiens and Bacopa. You can apply Bonzi on some of these crops; but at a much lower rate; on vinca and New Guinea baskets/large containers, consider drenching with ¼ ppm Bonzi only . . . . . .JUST ¼ ppm for good control (Figure 1). Control ‘Dragon Wing’ begonia with a ½ ppm drench.

Sprays:In general, 3 spray growth retardant applications/combos are emerging as the best late season options: B-9+Cycocel, Sumagic, and Topflor. I will discuss the benefits/disadvantages of each below. Disadvantages of PGR sprays compared to drenches is that bloom size can be decreased at higher rates and some impatiens bud opening may be slowed.

- B-9+Cycocel: General spray rates are 1250-2500 ppm B-9 + 250-500 ppm Cycocel. Works on most crops. Lowest residual activity (2 weeks). Little chance of overapplication (both PGRs are minimally root absorbed). Can be more costly than Sumagic application described below.

- Sumagic: General spray rates are 1-2 ppm. Works well on almost all crops but DO NOT spray on begonias or Bacopa; it can stunt them. Spray uniformity is very important and soil wetting with the PGR solution should be avoided. Sumagic sprays are now labeled for application on some vegetable transplants (tomato, pepper and eggplant) – follow label restrictions.

- Topflor: General spray rates range from 20-40 ppm. Topflor is expensive compared to the other PGRs. Therefore, it should be used primarily on vigorous nursery and perennial products that are hard to control. Topflor sprays are commonly used on canna lily.

- Cycocel: Cycocel is the PGR of choice for begonias; spray with 250-500 ppm. Cycocel is also effective for height control on Hibiscus (suggest 50 ppm; many new cultivars are more sensitive) and geraniums (250 – 500 ppm).

Final Recommendations: Consider spraying Sumagic on your crops finished in trays and pots up to 4” in diameter with 1-2 ppm Sumagic (start with 1 ppm). Consider drenching everything (except those identified

Figure 1. Vinca baskets held with a ¼ ppm Bonzi drench in TX (June-July). Baskets will stay this size for 1-2 months but keep blooming. Reduce rate further if you want less control (1/8th ppm). Only apply this rate when plants reach a marketable size!Photo: John Erwin, UMD

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Avoiding Botrytis BluesBy: Karen Rane

April is often the time for Botrytis outbreaks in greenhouse bedding plant production, and growers need to be prepared – especially when the market is slow and plants must be held longer than usual. Cool, wet weather and crowded flowering plants combine to make a perfect setting for this disease. A multi-pronged approach is needed to keep Botrytis at bay.

Botrytis is an insidious opponent – the fungus can colonize dead plant material like fallen leaf debris, and the spores are easily moved on air currents. Botrytis can infect virtually any plant species, and almost any plant part – stems, leaves, flowers, even fruit. Wounded or senescing tissues like old flowers are most easily colonized, but the fungus can also invade healthy tissue, causing spots, blights, stem cankers and plant collapse. In moist, humid conditions the fungus produces dark stalks with fuzzy gray spore clusters on infected tissue, giving the disease the common name of “gray mold”. These spores move in the tiniest of air currents. This is why it is so important to keep greenhouse floors and benches clean of plant debris and cover waste containers in the greenhouse to help reduce the sources of Botrytis infection. Deadheading – removing old flowers – also helps in this regard. Botrytis-infected petals can drop onto healthy leaves and cause leaf lesions.

Managing environmental conditions to reduce leaf wetness and humidity is very important in Botrytis management. High humidity enhances spore production, and free water is needed for spore germination and infection of plant tissues. Increasing plant spacing helps facilitate air movement and rapid drying of plant surfaces, but this may not be practical during times when shipping is delayed. Drip irrigation keeps foliage dry. If you must use overhead irrigation, schedule watering for early in the day to minimize leaf wetness. Horizontal air flow fans will help, and venting and heating the greenhouse at dusk can help decrease humidity as well.

Fungicides are the third factor in Botrytis management. Protectant sprays should be applied when conditions are favorable for Botrytis development. There are a number of fungicides effective in managing this disease – it is important to rotate products with different modes of action (as indicated by different FRAC codes) to avoid resistance development in Botrytis populations. Fungicides that are effective in Botrytis management include fenhexamid (Decree 50 DF, FRAC 17), iprodione (Chipco 26019, FRAC 2), chlorothalonil (Daconil and others,

Botrytis blight on geranium flowersPhoto: K. Rane, UMD

Botrytis leaf lesion on geraniumPhoto: K. Rane, UMD

below) finished in larger containers with 2-4 ppm Bonzi (2 ppm now, 4 ppm in May). Remember New Guinea impatiens, vinca and begonias are VERY sensitive to these compounds; use MUCH lower rates (see above) or simply apply 2500 ppm B-9 weekly as a ‘light’ control instead.

John Erwin is the new Chair in the Department of Plant Science and Landscape Architecture at the University of Maryland – College Park. He was formerly the Greenhouse and Nursery Crop Production Physiologist and Extension Specialist for the state of Minnesota and consults nationally with growers. His cell phone is 612-385-6863. You can email him at jerwin@umd.edu.

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FRAC M5), polyoxin D zinc salt (Affirm WDG, FRAC 19), azoxystrobin (Heritage, FRAC 7), fludioxanil (Medallion, FRAC 12) and combination products such as boscalid+pyraclostrobin (Pageant Intrinsic, FRAC 7+11), fluxapyroxad+pyraclostrobin (Orkestra Intrinsic, FRAC 7+11), and benzovindiflupyr+azoxystrobin (Mural, FRAC 7+11). Follow label instructions carefully for specific hosts and other cautions, especially if plants are flowering. Some products can cause damage to flowers on specific plant species.

Botrytis blight on verbena stemPhoto: K. Rane, UMD

Closeup of Botrytis sporulation on petunia stemPhoto: Purdue Plant and Pest Diagnostic Laboratory

Greenhouse Management With Cool 10-day Weather PredictionBy: Andrew Ristvey

Hopefully you all were able to move out the 1st crop from the houses and are transitioning into the new spring crop and the varieties you will be growing will most likely increase. The next 10 days are forecasted to be cool and cloudy, with rain distributed throughout the time period. Most of you have dealt with this before in the recent past with cool and wet springs.

Remember that your plant quality will depend on the environment that you provide your plants. Water use, nutrient availability and uptake, and growth are dependent on temperature. In order to save some money, you may be tempted to keep the temperatures low and wait out the cool, cloudy weather. If you are not concerned about a timeline and you can carefully manage water and nutrient applications, while monitoring substrate chemistry also, then maybe you can “weather” the next couple of weeks; or maybe the long-range forecast is completely wrong. Most have a timeline and cannot hold plants past the target production date.

Keeping temperatures well above the “base” temperature for growth is key to finishing plants on time. The base temperature is the minimal temperature needed to grow a crop. Temperature affects different crops differently, depending on each species’ base temperature.

During cloudy, cool periods, be sure to maintain proper temperatures to finish a crop on timePhoto: Andrew Ristvey, UME

The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by University of Maryland Extension is implied.

Read labels carefully before applying any pesticides.Photographs are by Suzanne Klick and Stanton Gill unless stated otherwise.

The University of Maryland Extension programs are open to any person and will not discriminate against anyone because of race, age, sex, color, sexual orientation, physical or mental disability, religion, ancestry, national origin, marital status, genetic information, political affiliation, and gender identity or expression.

This link (https://gpnmag.com/article/implications-base-temperature/) to Greenhouse Product News takes you to an article by Drs. Runkle and (MSU) and Blanchard (now at Syngenta) which has a list of different base temperatures for greenhouse crops. Cold tolerant plants like pansy or petunia can tolerate and grow in lower greenhouse temperatures than impatiens and vinca. The article suggests zoning greenhouses for efficient production of crops with different tolerances. The effect of temperature differential is more dramatic at cooler temperatures. For instance, a 5 °F difference is more pronounced on plant growth at or below 65 °F than a 5 °F difference around 75 °F.

Temperatures have an effect not only on plant growth but also nutrient uptake and disease. Cooler temperatures effect the way nutrients are available and are taken up. Plant uptake and use of nutrients like calcium which is taken up passively through the root tips, rely on warm ambient temperatures and relatively high vapor pressure deficits (low relative humidity) for effective plant transpiration.

Apart from phosphorus availability and plant uptake, nitrogen nutrition is also linked to temperature. Not only do winter conditions create substrate-based ammonium problems, but the way in which plants utilize nitrogen also lends to the difficulties. Plants take up both nitrate and ammonium nitrogen. Nitrate can be “safely” stored for later use by plants but ammonium cannot. During the process of ammonium uptake, the substrate becomes more acidic, lowering the pH of the media (another good reason to monitor media pH). Once inside the plant, ammonium nitrogen turns into ammonia. The ammonia needs to be immediately utilized through amino acid production or it becomes toxic. Photosynthesis is necessary to create carbon molecules used to sequester ammonia and make amino acids. Lower light levels and colder temperatures all contribute to slow growth and lower than normal levels of photosynthesis. This creates a deficit of carbon molecules for amino acid production, and no place for the ammonia to go, which eventually disrupts metabolism and respiration resulting in cell death. With higher concentrations and availability of ammonium and cool greenhouse conditions, a perfect storm exists for ammonium toxicity.

With cooler temperatures and higher relative humidity, media will become waterlogged from the lack of plant transpiration. This can create a poor, anoxic root environment promoting disease problems, not to mention the added media pH problems. Also the lack of evaporation of moisture from leaves may lead to environments that promote powdery mildew and botrytis.

With the new crop coming in and the cool temperatures and low light conditions expected in the next 10 days, manage your greenhouse temperatures accordingly. Maintaining temperatures to at least 70 °F during the day will ensure that all your crops are done when they will be needed. Interestingly, you should not lower your night temperatures too much either. Research has shown that large differentials in day/night temperature elongates plant stems. Keep your greenhouses warm in the coming weeks both in the day and at night (ideally 72 °F day/65 °F night). The expense of heating may be offset by the expense of associated problems with cool greenhouse environments. If you choose to grow cool, reduce fertilizing rates, use higher nitrate-ratio fertilizers, and monitor your substrates. If you have any questions about this or other nutrient management topics, feel free to contact me at aristvey@umd.edu.