Floriculture Greenhouse Update

Basil - Downy mildew

Tuesday, 17 August 2010 10:16

This information was summarized from a recent article by Rob Wick and M. Bess Dicklow, Plant, Soil and Insect Sciences, UMass Extension published in Floral Notes, 23(1), July-August 2010

During 2008 and 2009, Downy mildew (Peronospora species) of basil occurred throughout the east coast in epidemic proportions both in the field and in greenhouses. Although the downy mildew pathogen has been detected in basil seed; seed transmission is probably a rare event. Air-borne dissemination from infected plants is more likely.

Symptoms on the top of the foliage occur as yellowing as if the plants are nitrogen or nutrient deficient. When spores are produced, a characteristic gray, fuzzy growth on the underside of the leaves is evident. Symptoms of downy mildew on basil can easily be mistaken for nutritional deficiency.

The most important environmental factors favoring disease development are high humidity and extended leaf wetness. Cultural controls are directed at reducing the amount of time leaves remain wet by reducing plant density and watering early in the day. In the greenhouse, horizontal air flow can be increased and condensation reduced by fans. Removing and destroying severely infected plants can slow disease development.
Field trials conducted in southern New Jersey in 2009 determined that commonly-grown sweet basil (Ocimum basilicum) cultivars such as ‘Poppy Joe’ and ‘Nufar’ were the most susceptible to downy mildew. The least susceptible basils include the lemon and spice types such as O.x citriodorum and O. americanum  cultivars, “Lemon Std’, ‘Lemon’, ‘Lime’, ‘Spice’, ‘Blue Spice’ and ‘Blue Spice Fil’.

Few fungicides are labeled for herb plants. Copper products and phosphites are labeled for use on basil. It is the grower’s responsibility to read and follow label directions. The label is the law and any recommendations made here are superseded by the label.

At the University of Massachusetts, we are investigating methods to control this disease with biological control agents. We are interested in collecting live, infected plants.

If you think your basil plants are infected, please call or email Rob Wick, Dept. of Plant Soil and Insect Sciences; 413-545-1045, This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Photos: DM symptoms on top of Basil foliage, DM symptoms on underside of leaf

Robert Wick and Bess Dicklow, UMass Extension

Note: Downy mildew has also been observed recently on ornamentals in the Lamiaceae family, which includes basils (Ocimum spp), mints (Menta spp.), sages (Salvia spp) and other aromatics. According to Margaret McGrath, Cornell University, the coleus and basil downy mildew pathogens have now been demonstrated to be genetically different; therefore, these ornamental plants are not considered potential alternative host. However, there are many ornamental basils that are also hosts to pathogen affecting basil grown for use as an herb.
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension

Fact Sheet with photos and more details: Basil Downy Mildew (Cornell University)

Insect and mite pests during the dog days of summer

Monday, 02 August 2010 11:04

The hot, dry summer has resulted in outbreaks of western flower thrips, spider mites, potato leafhoppers,  and tarnished plant bugs have also been troublesome.  Smaller, black melon aphids also tend to be more prevalent in the summer and fall months.

Thrips
Thrips are pollen feeders, so as more plants are flowering, their populations tend to increase.  In the summer months, as grasses or nearby hay fields are mowed, thrips often enter greenhouses thru vents and open roofs. (It is often helpful to close the roofs if nearby areas are being mowed in the evening.)  They also may migrate to yellow flowering weeds such as galinsoga, so good weed control always helps.  

Here’s an example of how temperature speeds up their development:

Temperature:         Egg to Adult Development Times for Western Flower Thrips  
(On chrysanthemum leaves, 45-55% Relative humidity, Robb 1989)
59F            39 days
69F            26 days
77F            12.9 days
81F            10.2 days
80F            9.3 days
90F            10.7 days

With the increased development time, and emigration of thrips, it is very difficult to keep thrips levels at those maintained in the spring before plants bloom.  Sticky card counts vary depending upon the how attractive the plants are in the greenhouse to thrips, whether the plants are flowering, placement of cards, what stage of thrips is present, and migration of thrips into greenhouses.  Growers also need to relate sticky card counts to crop damage.  For example, high numbers of thrips may be present on the sticky cards in open roof greenhouses, but limited plant damage to garden mums may be observed.

Working grower action thresholds of 10 per card may be appropriate only during spring production.  Higher action thresholds may be more appropriate in the summer months for garden mum and poinsettias. (Woody plants such as roses and poinsettias are not hosts of impatiens necrotic spot virus).

If using chemical sprays, spray intervals need to be shortened to every 3-4 days.  For more information see, Thrips Management Plan and
Western Flower Thrips- Management on Greenhouse Grown Crops

Nematode application frequency needs to be increased in hot summer weather conditions. It's also important to apply enough moisture with the nematodes.  Sprays may dry out too rapidly in the high, dry environment.  One grower says he is now applying 500 gallons instead of 200 gallons of spray solution  to the same sq footage.  As with any biological control measure, they are most effectively used preventively in conjunction with good cultural practices for thrips control (sanitation, rigorous weed controls, etc).
For more information see the article:  Nematodes in Pest Management.

Fact Sheet: Field Grown Cut Flowers: Tarnished Plant Bugs

Leanne Pundt, University of Connecticut

Chrysanthemum White Rust

Thursday, 29 July 2010 15:31

Chrysanthemum white rust (Puccinia horiana) is a serious fungal disease of chrysanthemum. White rust can spread quickly in greenhouse and nursery environments causing severe crop losses.

Chrysanthemum white rust (CWR) is a quarantine significant pest in the United States; therefore, occurrence of this disease leads to state and federal regulatory action. In the last few years chrysanthemum white rust is occurring with more frequency in New England.  

The federal management plan for CWR includes the following measures destroying symptomatic plants, fungicide treatments on remaining asymptomatic plants, surveys of the surrounding area, and trace backs of infected stock. The full management plan can be viewed on the web on the USDA, APHIS website.

The symptoms of chrysanthemum white rust are very distinct. Light green to yellow spots up to 5mm in diameter appear on the upper surface of the leaf. These spots become brown and necrotic with age. Raised beige to pink pustules form on the underside of leaves beneath the spots. Pustules become white with age. Pustules are most common on young leaves and flower bracts but may form on any green tissue or the petals. Symptoms usually occur during cool, wet weather.

Infected plants may show no symptoms until environmental conditions encourage symptom development. If the warm, dry weather pattern this season in southern New England continues, symptoms may be delayed. Preventative fungicide applications, being used by some growers, will suppress disease development.

For a list of treatment options see the floricast by Margery Daughtrey, Cornell University . Syngenta has a bulletin on chrysanthemum white rust that can also be downloaded.

Note: Growers finding CWR are required to report the finding to their State regulator agency. There are on-going discussions concerning Chrysanthemum White Rust regulations. As a result, APHIS has established a Stakeholder Registry for those interested in receiving updated information related to these discussions. When registering, check Chrysanthemum White Rust  in Topics of Interest.

Paul Lopes, UMass Extension
With input from Margery Daughtrey, Cornell University