Bacterial Fruit Blotch of Watermelon

Nothing says summertime like slicing open a nice juicy watermelon! North Carolina produced 7,200 acres of watermelons valued at just over $24 million dollars in 2010. Most watermelons are grown in the sandy soils of the Coastal Plains or in the northeastern region of the state. 

We’ve seen quite a few watermelon samples at the Plant Disease and Insect Clinic this summer. Typically the most common diseases on watermelon in NC are Fusarium wilt, anthracnose, Cercospora leaf spot, gummy stem blight, and downy mildew. This summer we have received several samples with greasy lesions on the surface of the fruit. The disease: Bacterial Fruit Blotch or BFB caused by Acidovorax avenae subsp. Citrulli, or AAC.

Bacterial Fruit Blotch: Note greasy lesion and cracking (Photo: E. Lookabaugh)

AAC causes disease on most cucurbits, including watermelon, cantaloupe, muskmelon, cucumber and pumpkin. Bacterial fruit blotch was first reported in the U.S. in 1965 but did not receive much attention until 1989, when major outbreaks occurred throughout watermelon production regions. The disease now occurs every year with varying severity. 
 
AAC is a seedborne pathogen. Seedlings from infected seeds generally do not perform well. Water-soaked oily areas can be observed on the underside of cotyledons or other young leaves. Lesions expand along the veins and, in severe cases, infected seedlings collapse and die. Sometimes symptoms are inconspicuous or absent and the infected seedlings get transplanted to the field. There the bacteria are spread to healthy leaves and developing fruit by workers, on contaminated equipment, or in rain and irrigation splash. 

Water soaking on the underside of cotyledons (Photo: APS image library)

Once deposited, AAC infects through the stomata (tiny pores). Infections are most likely when a film of water is present on leaves or fruit, so rain, irrigation, and high humidity favor disease increase. The window for fruit infection is relatively short and lasts from just before flowering to about three weeks later. Once wax deposition begins on older fruit, the bacteria can no longer infect. Typical fruit symptoms include greasy olive colored lesions on the surface of the fruit. Under humid conditions, the lesions quickly expand. The lesions do not extend below the rind so the fruit is generally not affected at first. Over time, the rind begins to crack, allowing for secondary fruit rotting pathogens to invade.

Later symptoms: Note inconspicuous spots on foliage in  B (Photo: APS image library)

The most important way to manage this disease is through the use of certified disease-free seed. Plants produced from infected seed may not exhibit symptoms if conditions are unfavorable for disease. This means that asymptomatic watermelons may produce infected seed that can spread the disease. Certified seed is produced and tested with special methods that minimize the risk of seed transmission.

Managing bacterial fruit blotch once it is found can be problematic. Always keep seedlots together when planting seeds directly into the field or greenhouse. Monitor plants and remove infected seedlings. If one seedlot is contaminated, avoid spreading disease to healthy plants. Use drip irrigation rather than overhead irrigation to limit splashing. Plant spacing increases air flow and reduces humidity. Seeds from rotten fruit left unharvested in the field can produce infected volunteer plants the following year, which serve as sources of inoculum. Remove all volunteer plants to prevent production of secondary inoculum! 

Preventative chemical sprays applied before and during flowering prevent fruit symptoms from developing. There is no complete form of resistance available in watermelons, but varieties vary in their susceptibility. Varieties with dark-green rinds tend to be less susceptible than varieties with light-green rinds. Also triploid seedless watermelons tend to be less susceptible than diploid seeded watermelons. 


So, remember always start your watermelons from certified, screened seed. Don’t save watermelon seeds for next year – use them for a spirited seed spitting contest instead!

Tobacco Budworm

Tobacco Budworm

This great image is an adult tobacco budworm, Heliothis virescens, clinging to some unidentified stink bug eggs on a tobacco leaf in Nash Co. The photographer assumed that they were budworm eggs, but budworm eggs would be smaller and of a different shape, and I doubt a moth would risk damaging her eggs by hanging from them like that. In checking our records of insects on tobacco, I was surprised to find that out of several hundred samples, only 1 was of stink bug, some unidentified nymphs. The tobacco budworm is most often reported from tobacco and cotton in NC, but we also have records from geranium, petunia, rose, snapdragon, soybeans and even Fraser fir.


Post written by David Stephan

Time for Peanut Disease Control

Written by: Barbara Shew, Peanut Extension Specialist, NC State University

Leaf spots

Now is the time for peanut growers to begin foliar disease control programs. The first fungicide spray should go on when peanuts reach R3, or when about half the plants in a particular planting have at least one pod starting to develop. Most fields in North Carolina will be at or near R3 by now. In most cases, a group M (multisite) fungicide such as Bravo (chlorothalonil) or Tilt Bravo (groups 3 and M) should be used for the first spray in a 5-spray calendar leaf spot control program. You may be able to reduce the number of leaf spot sprays applied in a season by using the North Carolina leaf spot advisory after the first spray.

Example of a plant at R3 (very early pod stage) (Photo: B. Shew)

Southern stem rot

Southern stem rot usually starts at the crown of the plant or on stems touching the ground near the crown. Infected stems are the color of a brown paper bag. The white fungus growth is thick, stringy, or fan-shaped. Later, fungus structures about the size and color of mustard seed will develop.

Signs and symptoms of southern stem rot (Photo: B. Shew)

Southern stem rot is very active in hot, wet weather. Most peanut fields benefit from applications of fungicides that control stem rot during the hottest part of the season – usually from mid-July to late August. In North Carolina, we recommend starting stem rot control at the second leaf spot spray, or approximately two weeks from now. Fields at high risk for stem rot problems – those planted to a highly susceptible cultivar like Gregory, irrigated fields, fields or cultivars where heavy vine growth can be expected, and fields that have a history of stem rot and/or vegetable production – may benefit from using a soil fungicide for the first spray. Foliar fungicides effective against stem rot include Abound (group 11) and Provost (group 3). Artisan (groups 3 and 7), Convoy (group 7) and tebuconazole (group 3) are also effective but must be mixed with a foliar fungicide such as chlorothalonil or Headline (group 11). Fontelis (group 7), which recently was labeled on peanut, also is active against leaf spots and stem rot. Growers need to alternate fungicides from different groups or mix with a group M fungicide during the season to optimize leaf spot and stem rot control, and to avoid potential resistance problems. Remember that group 11 fungicides can only be applied twice in a 5-spray program. See the Agricultural Chemicals Manual for rates and other details. 

The new cultivar Bailey has moderate leaf spot resistance and good resistance to southern stem rot. The first spray can be delayed two weeks (R3+2) on Bailey. The new cultivar Sugg also has resistance to these diseases. Fungicide programs have not been tested as extensively on Sugg as on Bailey, but I believe that sprays can also be delayed on Sugg. Growers who chose to delay sprays on Sugg should be sure to scout carefully until we have more experience with this cultivar.

Begin scouting for Sclerotinia blight!

This week’s cool rainy weather has been highly favorable for Sclerotinia blight, so it’s time to begin scouting. Outbreaks of Sclerotinia blight can develop even in early July if weather is favorable and rows are within 6 inches of touching. It is important to check early for Sclerotinia blight because we have shown that the fungicide application made at the time of the first outbreak is the one most critical for control during the season. 

To scout for Sclerotinia blight, check several 100-foot sections of row. Early Sclerotinia infections often are found on limbs away from the crown. The fluffy white fungus is easiest to see early in the morning or after a rain. You must look inside the canopy to spot these infections. A 3 or 4 foot section of half-inch dowel or PVC pipe is a good tool for pushing vines aside for scouting. Infected stems develop a bleached appearance as the disease progresses. In fields with a history of Sclerotinia blight problems, apply a fungicide if you see active infections on an average of about 1% of the plants in a row. Continue to scout and monitor Sclerotinia advisories as the season continues. 

Early infect of Sclerotinia blight. Note fluffy fungus and bleached stems (Photo: Damon Smith)

Leaf spot and Sclerotinia advisories are available daily during the summer. Email Barbara Shew or your county agent if you would like to receive advisory emails. Advisories are also available on-line here 

Alert: Soybean Rust Found in Georgia!

North Carolina State University and Cooperative Extension

Steve Koenning Extension, Soybean Pathology Specialist,

and Jim Dunphy Soybean Specialist, Crop Science

ASIATIC SOYBEAN RUST FOUND ON SOYBEAN IN DECATUR AND COLQUITT COUNTIES IN GEORGIA

 

Alert: Soybean Rust July 6, 2012:

“Conditions have been quite favorable for spread of soybean rust in Georgia; we now have confirmed soybean rust on kudzu in Decatur, Miller, Baker, Brooks, and Lowndes Counties; soybeans in Decatur and Colquitt Counties. Most soybean acreage along Georgia Coastal Plain will be sprayed with fungicides at bloom-early pod set for rust and anthracnose and insecticides added for kudzu bug.”

Weather has not been conducive to build up of rust with the high temperatures in North Carolina, thus it may be premature for North Carolina soybean producers to spray soybean for rust. Be prepared, however, and remember that applications for Southern Rust of Corn may compete with fungicide/insecticide applications on soybean in terms of the numbers of aerial applicators and amount of fungicide available.

The current status of Southern rust, Asiatic soybean rust, and various other spreading pests in the U.S. can found here  or here

More information is available here 

Domark 230me is not listed in the Ag Chem Manual for corn at this time but is labelled at a rate of 4-6 oz. per acre.

When deciding to spray a fungicide, consider these factors in decision making!

1. When will the corn reach maturity – early maturing corn may escape infection and you might want to concentrate on the later maturing hybrids.
2. Strobilurins provide excellent control of rust in general but their residual activity is short compared to the triazoles. Additionally, Strobilurins have limited systemic movement in plants compared to triazoles such as Tilt, Folicur, Domark, Caramba, and Proline, and are thus less forgiving of less than perfect application.
3. Application costs may actually be greater than fungicide costs in many instances, so consider using the higher fungicide rate which will provide more residual protection and increase the likelihood that you can get by with one application.

Vigilance is the key word for 2012! Most sentinel plots have been planted and we will have about 14 this year. Storms following a track similar to Beryl’s could bring rust to North Carolina later in the year.

Resources for Soybean Rust in 2012: 

Some sources for more detailed information on Asiatic soybean rust and Southern corn rust are listed below: 

The North Carolina Agricultural Chemical Manual

Soybean Rust Management in the Mid-Atlantic Region

Late Blight of Tomato

Written By: Kelly Ivors, Extension Plant Pathologist, NC State University

During this 2012 season, late blight on tomatoes isn’t late. It was already confirmed in commercial potato and tomato fields on the NC coast weeks ago; however, we weren’t sure when we would see it in western North Carolina, where a substantial amount of tomatoes are grown commercially in the mountains. On July 5th, 2012 we confirmed late blight from foliar samples collected on July 3rd in a conventional field of tomatoes in Mills River, NC (Henderson County).

Without proper preventative measures and under the right weather conditions, late blight can completely defoliate and destroy a crop within two to three weeks. Due to moderate temperatures, frequent rainfall, and heavy morning dew during the growing season, late blight on tomatoes, caused by Phytophthora infestans, can be severe in the mountains of North Carolina, as well as in late plantings in the piedmont. Despite intensive efforts for over 150 years to control P. infestans, it remains one of the world’s most costly plant pathogens, concerning either direct loss and/or in the need for intensive use of costly fungicides. The recent spread of aggressive, Ridomil-resistant strains of this pathogen on tomatoes in NC has further aggravated the problem, making the pathogen much harder to control.

The pathogen attacks all above-ground parts of the tomato plant. The first symptoms of late blight on tomato leaves are irregularly shaped, water-soaked lesions (Figure 1); these lesions are typically found on the younger, more succulent leaves in the top portion of the plant canopy. During humid conditions, white cottony growth may be visible on the underside of affected leaves (Figure 2). As the disease progresses, lesions enlarge causing leaves to brown, shrivel and die (Figure 3). Late blight can also attack tomato fruit in all stages of development. Rotted fruit are typically firm with greasy spots that eventually become leathery and chocolate brown in color (Figure 4); these spots can enlarge to the point of encompassing the entire fruit.

Figure 1.The first symptoms of late blight on tomato leaves are irregularly shaped, water-soaked lesions.

Figure 2. During humid conditions, white cottony growth of P. infestans may be visible on the underside of affected leaves.

Figure 3. P. infestans can cause leaves to turn brown, shrivel and die.

Figure 4. Infected fruit are typically firm with spots that eventually become leathery and chocolate brown in color.

Causal Organism

Late blight of tomato is caused by the fungus-like organism Phytophthora infestans. The pathogen is best known for causing the devastating Irish potato famine of the 1840's, which killed over a million people, and caused another million to leave the country. Besides tomatoes, P. infestans can only infect a few other closely related plants including potato, petunia and related solanaceous weeds such as hairy nightshade. The pathogen is favored by cool, wet weather; clouds protect the spores from exposure to UV radiation by the sun, and wet conditions allow the spores to infect when they land on leaves. Nights in the 50's and days in the 80's accompanied by rain, fog or heavy dew are ideal for late blight infection. Under these conditions, lesions may appear on leaves within 3-5 days of infection, followed by white cottony growth soon thereafter (Figure 2). This white cottony growth is a sign of rampant spore (sporangia) production. Although spores may also be produced on tomato fruit, they are more commonly produced on leaves. Sporangia can be spread readily by irrigation, equipment, wind and rain and can be blown into neighboring fields within 5-10 miles or more, thus beginning another cycle of disease.

Figure 5.  Lemon-shaped sporangia of P. infestans attached to sporangiophores.

Disease Management

Host resistance
Plant resistance is not currently an integral component in late blight management for commercial production of fresh market tomatoes. However, new breeding lines resistant to some strains of P. infestans have recently been developed at the Mountain Horticultural Research and Extension Center in Fletcher, NC by tomato breeder Dr. Randy Gardner. A new campari-type (small fruited) variety called ‘Mountain Magic’ that has resistance to some strains of P. infestans, in addition to early blight, which should be available to growers in the following years.

Chemical
There are several diseases that attack tomato leaves and fruit in this region. Therefore, it is necessary to use a combination of different products in a spray program to optimize management of these diverse pathogens; products include strobilurins, mancozeb, and chlorothalonil. One consideration is that different products have different preharvest intervals (PHI). A product with a PHI greater than 1 day such as mancozeb (PHI = 5 days) cannot be used when harvests are done 2 or more times per week. Another important consideration is fungicide resistance management. For example, pathogens may develop insensitivity (resistance) to the strobilurins (i.e. Amistar, Cabrio, Quadris or Tanos) if these products are used too frequently.
The application of fungicides plays a significant role in the control of late blight of fresh market tomatoes; however mefenoxam resistant strains of the pathogen have been identified throughout the southeast. Fungicides containing mefenoxam are recommended only when weather favors disease development and resistant populations have not been identified in the area that season- usually this means mefenoxam can ONLY be applied for the first application when it is first found in a county. Resistance development to this active ingredient can be very rapid; use of this product after pathogen establishment in the area is not warranted or recommended.

Commercial growers in western NC should apply protectant products since controlling late blight preventatively is better than after infection. Before late blight infection occurs, mancozeb products such as Dithane, Manzate and Penncozeb work well early in the season before harvest (5-day PHI); chlorothalonil products (Bravo, Equus) work best during fruit growth (0-day PHI). In addition, several other chemistries such as cyazofamid (Ranman), fluopicolide tank mixed with a protectant (Presidio + chlorothalonil), and mandipropamid (Revus TOP) work well against this pathogen as foliar sprays.

Table 3-55 of the SE Vegetable Crop Handbook provides efficacy ratings against this disease. If you do not have a hard copy of the handbook, it can be found here

In addition, the NCSU tomato spray program website can provide advice on spray schedules and rates. The late blight products come into the spray program at week 9. Click here for more information