Dickeya dianthicola in the production of herbaceous ornamentals

During the month of March several producers submitted samples to the PDIC that turned out to be infected by the bacterium Dickeya dianthicola. See our full write-up on the Plant Pathology Extension Portal.

Bacterial Blight of Geranium

For the first time in several years, the Plant Disease and Insect Clinic received a sample of geranium with bacterial blight caused by Xanthomonas hortorum pv. pelargonii (formerly Xanthomonas campestris pv. pelargonii) from a North Carolina greenhouse. All geranium (Pelargonium) producers should be vigilant.

Typical leaf symptoms of bacterial blight of geranium.

In this case the submitted leaves showed small circular leaf spots and v-shaped necrotic lesions at the leaf margin. There were also edema-like bumps on the underside of the leaves. Petioles appeared healthy in this case, but in the advanced stages of this disease, stems can become infected, resulting in wilting. Note that with bacterial wilt caused by Ralstonia solanacearum there is no leaf spotting.

Close-up of upper leaf surface of Xanthomonas-infected geranium.

Water splash, tools, and handling are possible ways this pathogen can spread, and of course through infected propagative material (stock plants or buy-ins). Fortunately this strain of Xanthomonas does not affect any plants other than geranium, though it could survive in infected debris. Avoid overhead watering as much as possible. Keeping plants grouped by source can be helpful in containing and tracing any outbreak that occurs.

When Xanthomonas blight is confirmed in a geranium crop, the affected plants must be discarded, including the potting mix. Workers should avoid working among healthy geraniums after handling diseased plants. Asymptomatic (apparently healthy) plants next to diseased plants should also be destroyed, since they likely have populations of the bacterium on or in their tissues. The same goes for geraniums grown under hanging baskets containing diseased geraniums. All surfaces that had been in contact with these plants should be cleaned and then sanitized. This includes benches, tools, and pots. There’s a table with detailed information about sanitizers in the "Nursery Crops" section of the Plant Pathology Department Ornamentals page.

Preventive applications of a copper-based fungicide/bactericide, rotated with the biological control Bacillus subtilis, may reduce the spread of bacterial blight, but is not an effective strategy on its own. Use of clean stock and rigorous sanitation are the essential steps. For details on chemical applications, see p.448 of Table 10-11 in the 2015 North Carolina Agricultural Chemicals Manual.

For more information, see the following publications:
Bacterial Blight of Geranium by Gary Moorman of the Pennsylvania State University
Bacterial Blight of Geranium by M.B. Dicklow of the University of Massachusetts

A special thank-you to Dr. Mike Benson for reviewing this post.

TSWV in Chrysanthemum

A greenhouse-grown chrysanthemum was received in the Plant Disease and Insect Clinic on July 10th and diagnosed with Tomato spotted wilt virus (TSWV) by Emma Lookabaugh. Symptoms consisted of dark leaf spots, lateral curling of the leaves at some of the spots, and at least one stem lesion.

TSWV symptoms on Chrysanthemum

Although TSWV is the most commonly diagnosed viral disease here in the PDIC, it has been a long time since we've detected it on chrysanthemum from North Carolina. We have no records of it during the current millennium, but if memory serves there was at least one case back in the late 1990s. The current case does not constitute an outbreak, but should serve as a reminder to growers to take measures to prevent this disease.

A different sort of TSWV symptom on mum, from a different sample.

Tomato spotted wilt occurs on hundreds of field and crops, including peanut, tobacco, tomato, pepper, and potato, as well as on a wide range of ornamentals. In the last 6-1/2 years we have diagnosed it on the following ornamentals from commercial sources: African marigold, angel-wing begonia, calla lily, Cyclamen, Gaillardia, Gerbera, Senecio confusus, Lisianthus, Lobelia, Madagascar periwinkle, Sedum, and Stoke's aster. Its sister virus, INSV, is a frequent problem on many ornamentals.

Mottling and ringspot symptoms on TSWV-infected Senecio (left) and Stokesia (right)

Both TSWV and INSV can cause a wide range of symptoms, including mottling, ringspots, stunting, and necrotic leaf and stem lesions. Both are members of the genus Tospovirus and are transmitted by minute insects called thrips*. One curious fact about this transmission is that the virus is acquired by the insect during its larval development, but then the insect itself becomes permanently infected. Of course the virus can be brought into a greenhouse with infected plants, and could be perpetuated through vegetative propagation.

A thrips compared to the tip of a pin.

These strategies against TSWV (and INSV) are recommended for greenhouse flower production:

• Avoid growing vegetable transplants and flowers in the same greenhouse, and avoid growing plants of different ages together.
• Screen greenhouse vents and air intakes to exclude thrips from entering the greenhouse.
• Control weeds in and around the greenhouse. Many weeds are susceptible to tospoviruses and can serve as reservoirs of virus and thrips.
• Monitor greenhouses for thrips activity using blue or yellow sticky cards, with the top 2/3 of the card placed above the plant tops.  Use two cards per 5000 sq. ft. of greenhouse area.
• Use insecticides to manage thrips populations when necessary. Remove flowers from plants before treatment since the interior of flowers rarely get adequate coverage. It is important to note that some thrips populations have developed insensitivity to commonly used insecticides. In addition, no insecticide can completely eliminate thrips. Utilize the most effective chemistries wisely by rotating insecticides by mode of action (IRAC class) with each application, or at least with every generation of thrips. Always follow label directions and check that products are labeled for the intended crop. Details on insecticides for thrips management can be found in the NCSU Information Note on Western flower thrips and the University of Florida's thrips management information.

TSWV symptoms on Lobelia

If you suspect you have infected plants, we recommend having the diagnosis confirmed by a laboratory. Large growers with recurring problems may want to keep a supply of the simple lateral-flow ELISA tests on hand. Suppliers** include AC Diagnostics and Agdia. There is no cure, so all infected plants must be removed and destroyed. The potting mix of these plants should also be discarded, as this is where the thrips vectors pupate. Eliminate old stock plants as these are often sources of thrips and viruses.

More information about TSWV in the following crops is also available:
- peanut 
- tobacco
- tomato

Mike Munster and Steve Frank

*Grammatical footnote: The word thrips is both singular and plural.
**Mention of trade names and companies does not imply endorsement by North Carolina State University or the Plant Disease and Insect Clinic.

Leptoxyphium, an Unusual Sooty Mold

Ornamental sweetpotato leaves with sooty mold at the petiole/blade junction

The sooty mold Leptoxyphium on the underside of an ornamental sweetpotato leaf

This blog is going to be a bit more technical than many of my posts, but I hope you'll find it interesting. A recent sample of ornamantal sweetpotato leaves from a greenhouse showed dark fungal growth at the top of the petiole and on the upper and lower surface of the leaf, just at the point of petiole attachment. The colonies could be scraped off easily, which is typical of sooty molds. The fungus was sporulating freely, with conidia (asexual spores) produced in drops of liquid at the tops of dark synnemata (tiny columns of fungal hyphae).

Synnemata of Leptoxyphium sp. on sweetpotato leaf

Top of a synnema of Leptoxyphium, at 400x

Using Seifert & Okada's key to synnematous hyphomycete genera in the 2011 book "The Genera of Hyphomycetes", the identification was made to the genus Leptoxyphium. The name means "slender sword" in English, possibly referring to the shape of the synnemata, but it is interesting that there are also awl-shaped (subulate) cells around the fringe of the spore-bearing area. This genus is also described on pp. 777-782 of Stanley Hughes's 1976 paper "Sooty Molds" (Mycologia 68(4): 693-820). Leptoxyphium species are a tropical to subtropical sooty molds, and rather unusual in that they often grow in association with glands and glandular trichomes of plants, rather than on insect honeydew. The good news for the greenhouse producer is that while it is an asethetic issue, this fungus is not going to harm the plants.

Sample of the Week: Poinsettia scab

Rapid elongation of poinsettia stem infected
with Sphaceloma poinsettiae

Poinsettia scab, caused by the fungus Sphaceloma poinsettiae, was found on a sample from a commercial greenhouse this week.  It has been six years since the PDIC last diagnosed this disease in a North Carolina poinsettia crop. As the name implies, this fungus causes leaf spots and stem lesions, but the most noticeable effect is an abnormal elongation of the poinsettia stem. The purple leaf spots may develop a light tan center, and they sometimes have a yellow halo. The surface of the spot is characteristically puckered, which is best seen under magnification. An olive-colored, velvety layer of spores may be present on the spots and stem lesions. These spores are spread to other plants via water splash. Long-distance transport occurs on infected planting material. This disease cannot survive between seasons in North Carolina in the absence of a poinsettia crop. For a good summary of the disease, see the 2001 APSnet publication by Mike Benson et al. Growers should be sure they get clean stock and should scout points for leaf and stem symptoms. Keeping leaf wetness to a minimum will help reduce the advance of the disease. Apply azoxystrobin (Heritage), trifloxistrobin (Compass O), triflumizole (Terraguard), or triadimefon (Strike) to protect plants. An interesting side note is that while this fungus is a problem for poinsettia producers, it has been studied as a possible biocontrol agent for wild, weedy poinsettia relatives in the tropics. 


Special thanks to Dr. Kelly Ivors for contributing to this post.

Close-up of poinsettia stem showing scab lesions

Scab lesions on poinsettia leaf, caused by Sphaceloma poinsettiae

Sample of the Week: Cucurbit Downy Mildew on Greenhouse Cucumbers

This week’s sample is cucurbit downy mildew on greenhouse cucumber.  Cucurbit downy mildew affects all members of the cucurbit family: watermelons, cantaloupes, squash, pumpkins, and cucumbers.  Disease is favored by long periods of high humidity and mild temperatures, which unfortunately, describes most nights in the Southeast during the production season and in greenhouses year round.  Downy mildew occurs every year in North Carolina, and in recent years has become increasingly destructive on cucumber.  

Greenhouse Symptoms (Photo: PDIC Database)

The most obvious symptoms of downy mildew are small angular spots on the foliage, with older leaves generally being infected first. “Angular leaf spots” describe a type of symptom where the spots are defined by the boundaries of leaf veins, so that the shape of the spots is angular rather than round or blotchy. Downy mildew spots appear pale green to yellow as first, eventually becoming brown and necrotic with age.  

Angular Leaf Spots (Photo: PDIC Database)

During humid conditions, brown to purplish fuzzy growth can be observed on the underside of the foliage.  This downy growth is actually sporulation of the pathogen. 

Fuzzy Sporulation (Photo: Shawn Butler)

Dichotomously branched sporangiophores with lemon-shaped sporangia are visible when view through a microscope.  As infection progresses, leaves will eventually turn brown and curl upwards.  The leaves are the only part of the plant affected.  Downy mildew infections result in yield loss and misshapen fruits.  Damage from foliar infections also increases sunlight exposure on fruit and leads to sunscald.  

Cucurbit downy mildew is caused by Pseudoperonospora cubensis.  P. cubensis belongs to a group of fungus-like organisms called oomycetes (or water molds).  This group is also home to other aggressive plant pathogens including Phytophthora and Pythium.  P. cubensis is an obligate parasite, meaning it requires living host tissue to survive and reproduce.  North Carolina winter temperatures are too cold for the pathogen to overwinter, so it dies out every winter. Unfortunately, Southern Florida has the perfect combination of mild winters and wild cucurbit plants. The pathogen survives all year in Florida and inoculum builds up on wild plants.  Spores are windblown and can travel long distances on air currents. Summer weather events, like hurricanes, act as the perfect mode of transportation.  Like birds, downy mildew spores fly north for the summer and end up on our cucurbits! In this particular situation, the greenhouse plants probably became infected as spores from nearby field cucumbers blew into the greenhouse over the summer.

For more information on this disease, click here

For more information on controlling this disease, click here

For more information on forecasting this disease, click here 

Problem Pythium?

Have you been having a problem with Pythium root rot at your facility? Do you think that you may have resistant isolates? If so, you can contact the Plant Disease and Insect Clinic (link below) for more information about sample submission procedures.  I am going to be actively sampling North Carolina greenhouses for Pythium isolates.  If you would like us to sample your greenhouse or test your plants for mefenoxam resistance we might be able to add your facility to our list.  Please contact the clinic for more information. 

http://www.cals.ncsu.edu/plantpath/extension/clinic/

  

A Little Bit About Pythium

About Pythium

Pythium species are  “water molds” that produce swimming spores called zoospores. There are many species of Pythium.   Some species are saprophytes or weak pathogens that mostly decay dead root tissue.  Often, Pythium nibbles on feeder roots of plants. Under the right environmental conditions, however, some Pythium species become destructive pathogens that rot and kill the roots of plants, resulting in stunted growth or death.  The pathogen is favored by wet conditions, such as when media does not drain properly or when weather events prevent the soil from drying out completely.  Pythium can also be more problematic when the host plants are stressed.

             Pythium can be a problem on many annual and perennial hosts and is extremely destructive to greenhouse production of poinsettias and geraniums. Pythium can be introduced into greenhouses on infected plugs or infected plant material.  It can also be a year-round “resident” hiding on dirty plant containers, equipment, benches, or even in irrigation water, waiting for the conditions that favor the pathogen and disease development. 

Leaf curling associated with Pythium root rot on poinsettia

Stunted growth symptom of Pythium root rot on poinsettia

Managing Pythium Problems

Sanitation is very important in managing root rot because Pythium produces survival structures, called chlamydospores, that are able to survive for long periods of time on infected plant material or dirty benches and pots.  There are some fungicides that are effective against Pythium and other water molds (oomycetes).  Some of the most popular oomycete fungicides contain the active ingredient mefenoxam.  In recent years, greenhouse growers have seen an increase in oomycete resistance to mefenoxam, resulting in poor disease control.  Some good practices that limit the occurrence of fungicide resistance include:

• Time spray applications when the pathogen is weakest or most vulnerable to application
• Use a fungicide registered for your pathogen that is proven to be effective in controlled settings
• Alternate active ingredients in your spray programs
• Use only the labeled rates listed on the container and do not over-apply the fungicide
• Incorporate new fungicides into your spray program as they become available

Unfortunately, even when using good application practices, sometimes the pathogens still develop resistance. 

Research Goals

The purpose of my research is to determine which Pythium species are present in North Carolina greenhouses and to evaluate their abilities to be effective pathogens in the greenhouse environment.  This will involve morphological and molecular species characterization, pathogenicity assays, growth chamber studies, and checking for mefenoxam sensitivity.  

At the plant disease clinic, we routinely run root assays to determine the presence of Pythium.  Currently, we are not able to determine which species of Pythium is present and whether it is pathogenic or just saprophytic.  My research will allow the diagnosticians at the disease clinic the opportunity to expand their Pythium diagnosis to be more beneficial to the grower.  We will be able to tell the grower whether the Pythium we isolated from their plants is resistant to mefenoxam and if so, offer alternative control options.  

Fungicides are expensive.  Routine mefenoxam screening will help growers develop effective fungicide programs so they are not spraying money down the drain.