Phytophthora diseases of ornamentals

The plant pathogen Phytophthora attacks all parts of the host plants, causing root and crown rots, and foliar and stem blights. Many Phytopthora species are pathogens on hundreds of hosts, including many important ornamentals. Phytophthera infestans caused the great Irish potato famine in the 1840's, which resulted in the death of 1.5 million people, and the immigration of another million to the United States. Once established in a nursery or landscape, it can be difficult to manage.

Common diseases of ornamentals caused by Phytophthora spp. in south Florida include stem and leaf blight of English ivy; root, stem and leaf blight of pothos and peperomia; root, crown and leaf blight of spathiphyllum; stem rot of dieffenbachia; foliar blight and root rot on anthurium and mandevilla; and root and crown rot of liriope.

Symptoms and Signs
Foliar symptoms of root and crown rots resemble those of nutritional deficiencies or overwatering, and include chlorosis, wilting, and defoliation. The plant canopy fails to thrive, indicating a problem with the root system of the plant. The root system itself may be stunted, with areas of brown and reddish necrosis, especially on feeder roots. The root cortex sloughs off easily, leaving the root stele.

Phytophthora also causes stem and foliar blights. These are irregular areas of necrosis on the stems and leaves that often are black and can cover large parts of the canopy. Phytophthora foliar blight can be distinguished from Rhizoctonia blight by the absence of mycelia, although heavy production of sporangia can be seen in necrotic areas in white patches. Symptoms of stem blights and crown rots can be seen on the right. Phytophthora also causes cankers and root rots on several large tree species.

Causal Agent
Although Phytophthora resembles a fungus, producing hyphae and spores, it is actually an oomycete and not in the Kingdom Fungi at all. It belongs to the stramenopiles, a group of protists that also contains diatoms and brown algae. Other plant pathogenic oomycetes are Pythium and the downy mildews (Peronospora, Plasmopora).

Like fungi, Phytophthora has an asexual and sexual stage. The genus produces lemon-shaped asexual sporangia that can wither serve as new inoculum or produce zoospores. These zoospores have a tail, called a flagellum, that they use like a whip to propel it through a film of water. Zoospores spread disease by swimming from infected to healthy tissue. See below left for pictures of an intact sporangium and one that is expelling its zoospores.
Under certain conditions, Phytophthora will undergo sexual reproduction and produce a round, thick-walled oospore. These spores can survive harsh weather conditions and so allow pathogen survival through unfavorable seasons, making Phytophthora diseases a perennial problem.

Disease cycle and epidemiology
Phytophthora survives dry, unfavorable weather in infected tissue or soil as oospores or another thick-walled spore called a chlamydospore. These spores germinate when weather is favorable for disease development and produce sporangia that in turn produce zoospores. The zoospores swim through films of water on the host and soil and infect susceptible hosts. In general, cool, wet weather favors disease development, although some species are more active at warmer temperatures.

Disease management
Sanitation is important in managing Phytophthora disease, because once a soil is infested with the pathogen, it can be very difficult to get rid of. All nursery stock should be inspected for symptoms before planting and pathogen-free soil should be used. Disinfect used pots and tools with a 10% bleach solution. Alternatively, soil, tools and pots can be sterilized with steam treatments.

Do not over water plants, as the disease is favored by wet conditions. Overhead irrigation encourages development of foliar blight by splashing spores within water droplets, and providing films of water on the foliage that the zoospores can swim through. Keeping foliage dry can limit the development of foliar blights.

If Phytophthora is a recurring problem, plants should be regularly scouted for disease symptoms. Diseased plants should be rogued and destroyed so as not to increase inoculum loads. In areas of the landscape infested with Phytophthora, only species that are not susceptible should be planted.

Several fungicides are available for control of Phytophthora. These may be applied according to the manufacturer label as part of an integrated approach that also follows the cultural practices outlined above.

Disease of the month: Two new Bipolaris diseases in South Florida

From Dragon Fruit/Pitaya Project

Bipolaris fruit rot of pitaya
Pitaya (Hylocereus undatus), or strawberry pear, is a crop that is gaining importance in south Florida. It is commonly grown as a backyard ornamental for its fleshy, sweet fruit, and is also grown commercially on an increasing amount of acreage. Recently, rotten pitaya fruit from several commercial growers and back yard plantings have been submitted to the diagnostic clinic. This rot begins as small tan, circular lesions on the fruit surface and as the diseases progresses the lesions enlarge and turn black. Usually a black felt-like growth of the fungus can be observed on the lesions. Under ideal conditions (warm and humid), the fruit develop large areas of soft rot.

Causal Agent.
The lesions are caused by a fungus, Bipolaris cactivora. This species causes stem rot of cacti in California, Florida, and Europe, and causes a fruit rot on pitahaya in Japan. The fungus is easy to culture and produces black, felted colonies on PDA. Conidia are pale to medium golden brown and curved to straight, with 2-3 pseudosepta (the septations do not extend the entire width of the spores). Bipolaris is among several genera that were formally classified as Helminthosporium, a group of pathogens that cause a variety of leaf spots on ornamental plants, especially on grasses.

From Cordyline Project

Disease Cycle and Epidemiology.
The disease is most severe on mature and ripe fruit. While young stems are susceptible to B. cactivora, mature stems are relatively resistant to infection. Small tan lesions were produced by inoculating mature, wounded stems, but the lesions did not grow larger than 5mm. In the field, flowers also exhibit lesions with dark sporulation, and may serve as an inoculum source for developing fruits. While smaller lesions are seen on green fruit, rapid lesion growth and disease development occurs on ripe fruit after harvest.

Although the epidemiology of the disease on pitaya fruit has not been studied, Bipolaris rot on ornamental cactus is most severe between 75-91 F. In general, diseases caused by Bipolaris are favored by humid conditions. Inoculum sources include diseased plants in the field and crop residue. Conidia are most often spread by wind, irrigation and rain.

Management.
Currently there are no fungicide labeled for use on pitaya in Florida. Cultural management includes limiting canopy wetness by irrigating in the morning so plant surfaces can dry quickly throughout the day. Maintain a weed free planting and remove and discard of diseased plants (i.e. stems, fruits, and flowers) promptly when symptoms occur.


Bipolaris leaf spot of cordyline

Cordyline is a popular ornamental plant in South Florida landscapes. Last summer several cordyline samples were submitted to the clinic with elongated tan lesions with a red margin. Black sporulation was visible in the center of the lesions using a dissecting microscope.

From Cordyline Project

At greater magnification, these spores were identified as a Bipolaris species. Spores produced in the lab were inoculated on healthy cordyline plants, and after one week of incubation the same lesion symptoms occured, and the Bipolaris species was recovered from the new lesions. This demonstrated that the Bipolaris species was causing the disease.

The Bipolaris sp. causing this new leaf spot produces dark brown, felted colonies on PDA. Like B. cactivora, conidia are pale to medium golden brown, and are curved with 3-8 pseudosepta. We are currently working to identify the species name of this pathogen.

We are just beginning to investigate the epidemiology of this disease. Currently we are characterizing tolerance/resistance to Bipolaris leaf spot in different cordyline varieties, as well as the effect of temperature on disease development.

Managing cold damage

* 02/11/10: Another recent publication on cold damage - Click Here

After the cold snap over the holidays, there is a lot of concern about cold damage to plants in the landscape. To the left are pictures of a Dieffenbachia we left in our shadehouse before and after. As you can see, it didn't fair too well...

Just driving around the Redlands, widespread damage is apparent. A follow-up blog post about how to manage existing damage seemed timely, as many of you probably have questions about how to help your plants recover.

While damage done to foliage by cold temperatures is easy to spot because these leaves die and eventually drop off, the full extent of injury to trees and shrubs will not be evident until new growth begins.  

Pruning

Dead foliage is unsightly, and the first instinct is to prune out all branches with dead foliage immediately. But it is better to wait until spring to do any heavy pruning because you can use the pattern of new growth to determine which parts of plants are still viable. Pruning too early can lead to cutting back branches that may recover, and leaving other branches that will continue to dieback. The best course of action is to delay pruning until new growth begins. In the meantime, dead leaves may be removed or left to fall off naturally. When pruning in the spring, make sure to make cuts into living tissue just behind where dead wood is present. You can tell where the boundary between dead and living tissue is by where new buds are growing, or you can scrape the bark gently with a knife. Live tissue is green where dead tissue is brown or black.

Watering/ Fertilizing

While watering right after a freeze can help warm the soil, overwatering may decrease oxygen levels and encourage root diseases. If leaf canopies have been severely reduced, water requirements will be lower. Careful monitoring of soil moisture and judicious irrigation will benefit the plant until the plants regrow their foliage. Depending on the host, fertilizing trees and shrubs right after a freeze may encourage the growth of suckers and water sprouts, which may make reshaping plants difficult.

Sunburn

If sever defoliation occurs, direct sun exposure can cause sunburn on exposed twigs and branches. Whitewashing affected bark can protect it until foliage returns. A water-diluted of white latex paint can be sprayed or painted on defoliated branches.

Disease management

As warmer weather returns, damaged areas of the plants are highly susceptible to fungal and bacterial pathogens. A preventative spray of a copper fungicide applied according to the manufacturer label may help protect damaged plants from diseases.

Managing cold damage in palms

Cold injury to palms often takes a long time to appear, from many months up to one year. Often only the bud remains alive, and damage to the trunk can restrict water movement for years to come. To assess the full damage to palms, monitor emerging leaves in the spring and summer. If they appear brown or deformed, this may indicate damage to the bud, which the tree usually grows out of as the season progresses. Damage to the trunk manifests as a sudden collapse of leaves during the first periods of warm weather, due to reduced capacity for water to reach those leaves. If severe trunk damage occurs, then death of the palm is inevitable. To manage diseases on damaged palms, prune leaves that are completely dead apply copper fungicide according to the product label. Applying foliar fertilizer to the leaves can help new growth and recovery of injured palms in the warmer season.

For more information on cold damage in palms see:

• ENH-92: Treating Cold-Damaged Palms. 2003. Alan W. Meerow.

Managing cold damage to turf

Cold-damaged turf initially appears wilted and water-soaked, and eventually turns dark brown. Zoysiagrass and bermudagrass are more cold tolerant, while carpet grass and St. Augustinegrass are least tolerant to cold temperatures. Factors that contribute to cold damage include poor drainage, shade, close mowing height, excessive fertilization in the fall, and the buildup of thatch. Driving or walking on frozen turf will increase damage. Cold damage can resemble drought stress, and can lead to overwatering which in turn can encourage root stress and disease. Dormant turf needs less water, and so normal irrigation practices should be followed.

Further information on cold damage on turf can be found in:

• ENH80: Low Temperature Damage to Turf. 2000. L.E. Trenholm.

Listed below are several other useful publication on cold damage to plants. You can also visit our last blog post about cold damage for further information, photos, and links to extension publications:

• Dealing with Frost Damage. Dan Culbert. IFAS Extension feature article. February 27, 2007.
• Frost damaged plants may need pruning, but wait until spring. University of California, Agriculture and Natural Resources News and Information Outreach. Jan 22, 2007.
  

Abiotic Stress: Cold Weather Damage

Abiotic plant disorders are nonbiological factors, usually associated with the plant's environment, that affect plants adversely. Important environmental factors include temperature, moisture, soil pH, air quality, light regime, and plant nutrition. If one or more of these factors goes above or below the optimum range for a given plant species, plant growth might be abnormal or adversely affected. Abiotic disorders may also be caused by human activities, such as pesticide and fertilizer applications.

The biggest problem affecting plants growing in south Florida this month is COLD temperature. There are a lot of tropical plants in south Florida that can be severely damaged or killed by cold weather. Sometimes it doesn't even matter how cold the temperature gets, but the change in temperature that can be damaging. This even applies to watering with cold temperature on a hot day. The surface of a leaf in full sun can get very hot, so when you irrigate with water that is much colder (i.e. 40 degrees), the rapid change in temperature can damage plant cells. Refer to the previous blog post (i.e. mesophyll cell collapse) for further information.

Anytime there is a threat of cold weather in south Florida, one needs to take precautionary measures to protect subtropical and tropical plants from exposure. The easiest and likely most common way to guard landscape and garden plants against frost is to use some type of covering. Old blankets, sheets, towels and strips of plastic mulch all seem to work fine. When covering plants, drape them loosely and secure the covering to prevent it from blowing off. Rope ties and stakes, rocks, or bricks are items that can be used. Lighter material like plastics and sheets can be placed directly over the plants, but be careful with heavier covers that may crush plants due to the weight. It’s best to cover plants late in the afternoon or early evening before the onset of freezing temperatures and then to remove the covers late morning when the sun is out and the temperature is above freezing. Plants should be well watered the day before a frost or freezing temperature is predicted. Research has shown that wet soil holds more heat than soil that is dry. The best way to know what type of precautionary measure you should take for your plants is knowing their individual needs. The more you know the better off they will be.

Links to additional information on cold damage:

Cold Protection on ornamental plants:

http://edis.ifas.ufl.edu/mg025

Cold protection tool kit:

http://fawn.ifas.ufl.edu/tools/coldp/evaporative_cooling.php

Low temperature damage to turf:

http://edis.ifas.ufl.edu/lh067

Cold protection methods:

http://www.plantstress.com/Articles/cold_m/cold%20protection.pdf

What to do after a freeze:

http://trec.ifas.ufl.edu/doc/extension2010/What-to-do-after-the-freeze-for-trees-in-the-home-landscape-2010.pdf

Photos of cold damage:

Clinic Update 12/01/09

From TREC CLINIC Blog

From TREC CLINIC Blog

We had a pretty eventful past couple of weeks. Dr. Palmateer and Tara Tarnowski traveled to Puerto Rico to meet with Dr. Consuelo Estevez de Jensen , director of the University of Puerto Rico Plant Diagnostic Clinic, which is located in Juana Diaz. We are currently collaborating on a project to investigate more sensitive PCR methods for molecular diagnostics. We spent a day at the experiment station in Juana Diaz, trying out a new PCR method. We also travelled to Mayaguez to visit the Plant Pathology department at the Mayaguez Department, and visited with Dr. Brian Irish at the USDA-ARS station. He showed us their extensive collection of cacao germplasm, as well as many other neat tropical plants they have at the station. See photos to the left of the University of Puerto Rico experiment station in Juana Diaz, Dr. Estevez de Jensen in the USDA cacao collection, the biggest baobob tree on the island (also at the USDA station), and Dr. Palmateer appreciating one of the many breathtaking views on the island.

From TREC CLINIC Blog

From TREC CLINIC Blog

On October 13th, a group of agricultural scientists from Germany visited TREC. We showed them around the clinic and our tropical fruit collections. They then toured several local nurseries. The whole group seemed to enjoy seeing all the tropical fruit and foliage crops we have here in south Florida.

From TREC CLINIC Blog

Finally, on October 20th, the latest group of Master Gardeners from right here in Miami-Dade county came through for a tour. The were enthusiastic about everything we showed them, and asked all sorts of questions. We had fun looking at several disease samples together and testing their diagnostic skills. All of them did a pretty good job of distinguishing between symptoms and signs, and were really excited to see vascular discoloration, bacterial streaming, and several photos of fungal spores taken from the samples we were looking at. What a fun group!

From TREC CLINIC Blog

Disease of the Month: Physiological disorders of tropical foliage plants

Physiological disorders of tropical foliage plants

This month we are taking a look at two physiological disorders commonly seen on foliage plants here in south Florida during the colder months. Physiological disorders are not caused by pathogens, but by environmental factors instead. Because these disorders are not caused by biotic agents, they are not infectious and will not spread.

Oedema (edema) occurs when roots take up water faster than it can be used by the plant or transpired through the leaves. Water pressure builds up in the mesophyll or internal cells of the leaf causing them to enlarge and form tiny swollen blister like areas. The swollen blisters are usually round, and can become corky with age. The tissue under the surface of the lesion, however, often remains green. Oedema lesions can resemble insect galls or scale insects, but scales can be scraped off of the plant surface while oedema blisters cannot.

See above for a photo of oedema on orchid, and below for oedema on sapodilla (front and back of leaf).


Oedema is most common during winter months in south Florida (November-March) especially during extended periods of cooler, cloudy weather. The cooler air temperature slows transpiration and water evaporation by the leaves, and so excess water in the warmer soil is absorbed by the roots faster than it is lost from the leaves. Several tropical plant species are very susceptible to oedema. For example, Ixora, Peperomia, and some orchids may show symptoms of oedema any time throughout the year due to favorable conditions (i.e. excessive water, low air movement, etc.).


Oedema lesions are permanent, but under favorable growing conditions, affected plants will produce healthy new growth. Although it may be tempting to remove affected leaves as soon as lesions appear, these leaves are still able to photosynthesize and so removing them may be harmful to the overall health of the plant. Once sufficient new growth is present, the older blistered leaves can be pruned out. The most important step in preventing oedema is to make sure plants are not over watered, especially during cooler weather. Water in the morning, as daily temperatures are rising. Spacing plants further apart to allow more air flow on leaf surfaces can also reduce oedema. For potted plants, avoid accumulation of water in saucers and use well-drained soil mix.

Mesophyll cell collapse is another physiological disorder that occurs after exposure to low water or air temperatures. The low temperature directly damages the mesophyll cells of the host, but symptoms may not appear for 6-8 weeks after damage occurs. Symptoms are localized or large areas of sunken or yellow spots on the foliage. These areas may become brown and necrotic with age, and saprotrophic (not pathogens) fungi may colonize the dead tissue. New leaves are more susceptible than older leaves. On orchid species, temperature of 35-45°F and water that differs by more than 25°F from leaf tissue can cause mesophyll cell collapse. Some studies also suggest that extreme fluctuations in day/night temperatures may lead to the condition.

Mesophyll cell collapse can be difficult to diagnose because symptoms develop many weeks after the damage occurs. If you suspect that the symptoms you are seeing are due to mesophyll cell collapse, you can check temperature records for the area or greenhouse for the last six weeks and look for cold temperatures or extreme fluctuations in high/low daily temperatures. Also, the pattern of symptoms seen can be clue as to whether the damage is due to mesophyll cell collapse. If the damage is due to a single cold temperature event, symptoms will be seen on a few newer leaves, but growth developing after symptom development should be healthy.

Like oedema, damage from mesophyll cell collapse is permanent, but several steps may be taken to prevent or minimize future damage. When watering plants, take care that water is not too cold (more than 25°F/4°C different than plant tissue). Sensitive plants may need extra protection from cold temperatures (heaters and physical protection from cold winds).

Several IFAS extension publications exist for these two physiological disorders as well as other disorders caused by environmental factors:

Publication #PP265 Physiological Disorders of Orchids: Mesophyll Cell Collapse. R.A. Cating and A.J. Palmateer.



Publication #PP244 Physiological Disorders of Orchids: Oedema. R.A. Cating, A.J. Palmateer, C.M. Stiles, P.F. Harmon and D.A. Davison.



Publication #PP248 Guidelines to Idenitification and Management of Plant Disease Problems: Part 1. Eliminating Insect Damage and Abiotic Disorders. Monica Elliott, Ken Pernezny, Aaron Palmateer, Nikol Havranek.

Also, here is a thorough treatment of abiotic diseases on houseplants from the American Phytopathological Society:

APSnet Feature: Non-infectious diseases of common houseplants. January 2001.

BLOG POST 11/02/09

We've been busy this last couple of weeks, with 24 samples to diagnose and experiments underway. On October 16, we had a group of students in a tropical cropping systems class from Gainesville come down to TREC for a visit, and we talked about the ornamental industry here in Florida, as well as the function of the diagnostic clinic. It is always astonishing to read the statistics about how large the nursery industry is--it has an economic impact of $2 billion dollars a year, and provides about 40,000 jobs!

Dr. Palmateer gave a plant pathology training at the Miami Dade County Extension for the Master Gardeners. He was impressed at how interested and engaged the audience was.

Our newest dieback of Eugenia project is underway, we are testing different fertility and irrigation rates to see how they influence disease development. We currently have seventy healthy plants in our greenhouse, which we plan to inoculate next week, and then monitor for disease development. Our hope is that this experiment will provide useful information for growers about how to manage the disease with cultural methods.



Have a look at the Clinic website if you haven't already. You can find helpful information about the proper way to submit a sample (including a video demonstration), as well as contact information for Clinic staff.

Vegetable season is starting, so we expect to see more vegetable samples coming in...