Downy mildew of cucurbits: identification and management By Dr. Shouan Zhang UF/IFAS/TREC

Downy mildew is one of the most economically important diseases of cucurbits, especially in areas with high humidity and rainfall such as South Florida. This disease occurs on cucurbits including cantaloupe, cucumber, pumpkin, squash and watermelon. Downy mildew is an annual disease problem on pumpkin and squash in eastern and central US states for many years.  A tremendous breeding effort in the mid-twentieth century resulted in adequate control of this disease in cucumber production without fungicide application. However, downy mildew has become one of the most important diseases in cucumber since 2004.  The resurgence of the devastating disease has caused substantial economic losses to cucumber growers, and it continues to be an important disease problem in other cucurbit crops that significantly impacts the cucurbit production. Because the production of cucurbits exists year-round throughout the state of Florida, downy mildew is endemic and occurs nearly every growing season to some degree.

Fig. 1 Damage to cucumber plants by downy mildew in the field. 

Look out for the symptoms

Downy mildew begins as small water-soaked lesions on the underside of leaves. The lesions are usually limited by small leaf veins, giving the lesions an angular appearance. It is caused by an oomycetous pathogen called Pseudoperonospora cubensis. Under high humid conditions, a layer of gray-brown to purplish-black fluffy stuff appears on the underside of the infected leaves.  Inspection with a microscope of the presence of the acutely and dichotomously branched sporangiophores (tree-liked structure) bearing lemon-shaped sporangia is characteristic of the downy mildew pathogen. Leaves will turn necrotic and curl upwards within days if the weather condition favors the pathogen. Therefore, scouting for the disease is critical. 


Symptoms of downy mildew vary on watermelon and cantaloupe, and the lesions are not always angular and often associated with an upward curling of the leaves. The pathogen of downy mildew does not affect fruits of cucurbit crops. However, it can result in significantly reduced yields and deformation of fruits especially in cucumber.  In addition, increased exposure to direct sunlight due to early defoliation causes more sun-scalded fruits in watermelon and winter squash.

Fig. 2.  Symptoms of downy mildew on upside (left) and underside (right) of cucumber leaves. Note the angular and chlorotic lesions on infected leaves.  

(http: www.cals.ncsu.edu/plantpath/extension/fact_sheets/Cucurbits_-_Downy_Mildew.htm)

The mystery behind downy mildew

Pseudoperonospora cubensis is an obligate parasite that requires live host tissues in order to survive and reproduce. The pathogen must overwinter in an area that has mild weather in winter season, e.g. South Florida, and where cucurbits are present.  The spores are dispersed via wind or splash water to neighboring plants and to other fields by wind or irrigation water.  Symptoms appear several days to nearly two weeks after infection.  


The pathogen favors cool and moist weather.  Therefore, downy mildew is common during winter growing seasons in South Florida. Optimum conditions for sporulation are 15-25°C (59-77°F) with 6 to 12 hours of water film on the leaves such as morning dew. Sometimes temperatures during daytime are not favorable for this pathogen, however, nighttime temperatures may be ideal.

Fig. 3.  Symptoms of downy mildew on upside (left) and underside (right)

of butternut squash leaves. Note the gray-brown to purplish-black ‘down’ on the underside of the infected leaves.

Fig. 4.  Symptoms of downy mildew on leaves of cantaloupe (left) and watermelon (right). Note irregular lesions and turn brown to black in color.

(http: www.cals.ncsu.edu/plantpath/extension/fact_sheets/Cucurbits_-_Downy_Mildew.htm)

Downy mildew management

Resistant Cultivars

Host resistance is most economically important and effective component in integrated disease management and should be utilized whenever possible. Resistant cultivars have been developed for cucumbers and cantaloupe, and to a lesser extent for squash and pumpkin.  Though downy mildew has been severe on resistant cucumber cultivars in recent years, they are more effective than susceptible cultivars in delaying infection.

Early Detection

Early detection of downy mildew is critical for preventing cucurbits from the damage of this disease due to its rapid and destructive nature. Many growers lost huge values of cucurbits to downy mildew as they waited until symptoms of the disease were clearly seen before initiating sprays. Following early detection, immediately applying preventative fungicides is imperative for the control of this destructive foliar disease. A downy mildew forecasting system (http://cdm.ipmpipe.org/) has been established to assist cucurbit growers in timing their fungicide applications for maximum benefit.  The system documents the disease outbreaks and provides a risk assessment for future outbreaks in regions where cucurbits are grown.

Chemical Control

It is unlikely to achieve satisfactory control of downy mildew without the use of fungicides. Many growers heavily rely on fungicides for downy mildew control due to the aggressive and destructive nature of this disease. Both protectant and systemic products should be applied. Fungicides are effective in downy mildew control when applied prior to infection and continually sprayed at 5- to 7-day intervals. Previcur Flex, Tanos, Ranman, Reason, Presidio, Aliette and Quadris are examples of fungicides for control of downy mildew in Florida.  The products should be applied in a program to prevent resistance development in the pathogen population by rotating with fungicides of a different mode of action. Protective fungicides such as chlorothalonil and mancozeb should be incorporated as mixing partners. All fungicides should be applied according to the manufacturer's label. 

Shouan Zhang is a vegetable plant pathologist based at the IFAS Tropical Research

and Education Center, University of Florida in Homestead, FL. 

He can be reached at:

EMAIL: szhang0007@ufl.edu

PH: (305)246-7001 ext. 213.

Bacterial blight of Ficus Caused by Xanthomonas

Species of Ficus are common tropical foliage plants used for interior decoration and in landscapes throughout warm climates such as South Florida. Ficus species used as foliage plants include F. altissima, F. benjamina, F. binnedijkii, F. elastica, F. lyrata, F. microcarpa, F. pumila, F. retusa, and F. rubiginosa among others . Florida has led the nation in the production of foliage plants, accounting for more than 55% of the national wholesale value since the 1960s (Chen, J., R.J. Henny, and D.B. McConnell. 2002)

Hot, humid, and rainy conditions (typical summer in South FL) are highly favorable conditions for plant disease. Xanthomonas is a bacterial plant pathogen that thrives under these conditions and is spread very effectively in water, especially windblown rain. Once cells of the bacterium come into contact with the plant they enter through wounds or natural openings such as stomata or hydathodes. Once inside the plant they can move systemically causing a severe blight of the leaves.

Xanthomonas campestris pathovar fici is reported to be the causal agent of bacterial blight affecting species of Ficus. Host studies indicate that several Ficus species and cultivars are susceptible including F. benjamina , F. buxifolia, F. triangularis, F. mexicana, F. maclellandii ‘Alli’, F. retusa ‘California Nitida” and ‘Green Gem’ and F. Grennisland (Chase and Henley 1993)

Symptoms:

On young plants initial symptoms appear as small, water soaked, circular lesions with irregular borders near the leaf margin (Fig 1)

After 7 to 14 days of initial infection, lesions enlarge, coalesce and typically cover large portions of the leaf area (Fig 2)

Premature senescence and leaf drop is common. When young plants (liner stage) become infected the disease can be fatal (Fig 3)

Older plants can also become infected and show a range of symptoms. Spots on the upper surface of leaves are brownish-black with a chlorotic halo (Fig 4) and leaf tissue appears water soaked on the opposite side (Fig 5)

Management:

Be certain to monitor irrigation and avoid watering late in the day or evenings. If using overhead irrigation water in the morning, so that the foliage has time to dry before evening. Allowing the foliage to dry will reduce spread of the bacteria. Using micro jet or drip irrigation minimizes moisture on the foliage and reduces Be sure to surface disinfest all pruning tools. Good sanitation is crucial for managing bacterial pathogens.

As a preventative measure apply one of the following labeled pesticides according to the manufacturer. If copper based fungicides have never been used be certain to test on a small number of plants before applying to the entire crop. Be careful to use appropriate rates of copper compounds, with water at pH 6.0 or 6.5, since copper toxicity symptoms can look similar to those caused by the bacteria.

Current Research:

The current Xanthomonas outbreak on Ficus elastica has negatively affected many tropical foliage nurseries throughout Miami-Dade County. Plant losses are reported to be in the hundreds of thousands, so this is most definitely becoming an economically important disease.

The Palmateer lab is currently conducting research to 1) determine if Xanthomonas isolates from Ficus elastica are different from those previously reported to cause disease on other Ficus species; 2) conduct host range trials to see if other (i.e. Strelitzia spp., Ficus spp., Cordyline sp., Anthurium sp., Syngonium sp., and Dieffenbachia) popular foliage plants are susceptible; 3) evaluate the impact of temperature and light on disease; and 4) conduct bactericide efficacy trials.

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: