Agent Training in Plant Disease Diagnosis Coming Soon!

The Plant Disease and Insect Clinic and the Department of Plant Pathology announce an introductory County Agent training in plant disease diagnosis.

The training will consist of two live Elluminate sessions covering the basics of plant disease diagnosis followed by an all-day hands-on workshop. The Elluminate sessions are open to everyone. Seating for the workshop is limited to 25 North Carolina Cooperative Extension Agents. There is no fee for this training.  More information coming soon!

May 31, 2012 10:00 a.m. (Webinar)

 

Introduction to Plant Disease Diagnosis: plant diseases and the diagnostic process

June 5, 2012, 10:00 a.m. (Webniar)

 

Signs and Symptoms: recognizing  and describing disease symptoms and pathogen signs

The Plant Disease and Insect Clinic: collecting and submitting a sample; using the database; interpreting diagnostic results

June 7, 2012, 8:30 a.m. to 4:00 p.m

 

Plant Disease Diagnosis: hands-on workshop

Participants will examine plant diseases and disorders on live material in the laboratory and in the landscape, and will learn simple diagnostic techniques that they can use in the field or at their office. Agents will tour the PDIC and “walk through” the process of diagnosing a clinic sample. Lunch, breaks, and educational materials will be provided.

Be on the look-out! Garden diseases to watch for in May and June

With summer swiftly approaching, we are seeing quite a few more diseases out and about in the landscape and in the home garden. We wanted to take a few minutes and go over some common diseases you should look out for in your own vegetable garden. 

Tomato Spotted Wilt Virus 

A few days ago we received the second TSWV sample of the season on greenhouse tomato. TSWV is a virus spread by at least 7 different kinds of insects called thrips. TSWV has a very broad host range that includes a variety of ornamental plants, along with tomatoes, peppers, tobacco, and peanuts. Early symptoms of this disease include cupping or bronzing of the foliage and pale ring-spots/ mottling on the fruit. As the disease progresses, necrotic spots/lesions can be seen on the foliage, stems, petioles, and fruit. Infected plants are usually severely stunted and new growth is often deformed. Infected plants will not recover and should be removed from the garden. Detailed information can be found here: 

Early TSWV symptoms on tomato (Photo: E.C. Lookabaugh)

Ringspot symptoms on pepper (Photo: E.C. Lookabaugh)

Symptoms on tobacco (Photo: E.C. Lookabaugh)

Fruit Symptoms (Photo: F.J. Louws)

Septoria Leaf Spot of Tomato 

Septoria leaf spot is a very destructive fungal disease of caused by the fungus Septoria lycopersici. Usually this disease shows up on the lower foliage after the first fruit set. Necrotic spots can be seen on the foliage. As the spots age, the centers turn gray and tiny black dots (fruiting bodies of the fungus) can be seen in the center of the spots. The fungus survives the winter on infected tomato debris or nearby weeds. Controlling this disease in the home garden can best be achieved by removing all crop debris at the end of the growing season or by tilling it under the soil. In commercial situations, control can be achieved through the use of resistant cultivars and fungicides. 

Septoria foliar symptoms (Photo: E. C. Lookabaugh)

Southern Blight 

Southern blight is a serious and frequent disease in the Piedmont and Coastal Plain regions of North Carolina. This disease is caused by the fungus Sclerotium rolfsii, which attacks many vegetable crops including tomato, bean, cantaloupe, carrot, pepper, potato, sweetpotato, watermelon, and several field crops such as peanut, soybean, and tobacco. This disease is easily recognized by the white fan-shaped growth of the fungus at the base of the plants. Over time, tiny round tan to brown sclerotia are formed on soil and infected plants. These sclerotia can survive in the soil for MANY years. Rotation is not very effective because this pathogen has more than 1,000 reported hosts. Corn and some other members of the grass family are not hosts and are safe to plant in problem areas. In gardens, planting on a raised bed filled with sterile soil is the best way to avoid contact with native soil that may contain the pathogen. The disease is more active in warm, wet weather and can be seen every year in North Carolina. Watch for a more comprehensive post on this disease coming soon! 

Southern Blight (Photo: Kurt Taylor)

Southern Blight up close (Photo: F.J. Louws)

Southern Bacterial Wilt 

Here in North Carolina, southern bacterial wilt is one of the most commonly diagnosed diseases of tomatoes in the home gardens and production fields. This disease is found throughout the Piedmont and Coastal Plain regions of the state. It is caused by the soilborne bacterial pathogen Ralstonia solanacearum, and is most commonly found on tomatoes, peppers, potatoes, tobacco and other members of the nightshade family. Brown discoloration of the vascular tissue in stems and leaves is a distinctive symptom of bacterial wilt. The discoloration is caused by bacteria colonizing the plant’s vascular tissue, plugging it up. The plant loses its ability to conduct water, which results in yellowing and wilting, especially during the hottest part of the day. Infected plants quickly collapse and die. Diseased plants should be removed and susceptible species should not be planted back into infested areas. More detailed information on this disease can be found here: 

Complete collapse caused by bacterial wilt (Photo: F.J. Louws)

Vascular discoloration (Photo: M.J. Munster)

Root-knot Nematodes 

Root-knot nematodes (Meloidogyne sp.) attack a wide variety of vegetable and field crops, including tomato, peanuts, spinach, carrots, and many others. Usually root-knot nematodes are more common in eastern North Carolina where we have more sandy soils. Symptoms are worse is hot, dry summers. The most obvious symptoms are galls and swellings on the roots and stunting and yellowing of the above-ground portion of the plants. Control can be achieved through the use of resistant cultivars and crop rotation.

Root-knot nematodes on tomato, notice galls on roots (Photo: F.J. Louws)

Female nematodes under dissecting scope (Photo: F.J. Louws)

Plug Up Your Nose! Bacterial Soft Rot Arrives in the Clinic

There is nothing worse for clinic diagnosticians than receiving a package that smells before it’s even opened. If we are really unlucky, the smell is the pungent aroma of a bacterial soft rot sample. Last week, we were unfortunate enough to open one of these smelly packages to find a slimy mess of a tomato plant with a bad case of soft rot.

Bacterial Soft Rot on Tomato (Photo E.C. Lookabaugh)

Bacterial soft rot is easy to diagnose from the soft rot symptoms, and the unique, fishy aroma. It is caused by Pectobacterium carotovorum subsp. carotovorum (formerly known as Erwinia carotovora subsp. carotovora). These bacteria have a very wide host range and can also be a significant problem on potato and cabbage. Soft rot bacteria are ubiquitous and are found in relatively low numbers throughout most greenhouse production systems. 

Soft rotting bacteria are often called “opportunistic” pathogens because they are not able to directly penetrate host tissue – they need free water and a wound or other opening for infection to occur. Mechanical injuries, damage caused by insect feeding, or previous infections by pathogens like Phytophthora and Pythium allow soft rotting bacteria to enter the plant. Once introduced into an infection court, these bacteria quickly multiply to very high numbers. Soon after infection, a water-soaked lesion can be seen around the point of entry. As the bacteria multiply, they produce enzymes that quickly break down succulent host tissue. In the case of tomatoes, the soft inner tissue (pith) of the stem breaks down when infected. When the pith is lost, the stem becomes hollow and collapses. Soft rot can progress very rapidly under warm, wet conditions. Within 48 hours, soft rotting bacteria can turn a plant to mush.


The major means of controlling this disease is sanitation. The bacteria do not move through air but can spread mechanically on dirty equipment or on workers’ hands. Infected plants should be removed and destroyed. Mechanical equipment and pruning tools should be properly sanitized with a bleach solution. Workers should wash their hands thoroughly before working and after touching diseased plants. Avoid pruning, transplanting, or harvesting in wet weather. This disease is usually not important in field-grown tomatoes, but can become problematic after heavy rainfall or damage from hail storms. Avoid injuring fruit during harvest. Water used for washing tomatoes in packing houses should be treated to kill bacteria. However, washing infected tomatoes will not stop the disease from progressing; it will only serve to contaminate the water source. Avoid planting tomato in rotation with other host species.

Soft Rot on Cabbage (Photo: PDIC Database)

Alert: Stripe Rust of Wheat

We have found wheat stripe rust on a farm in southern North Carolina. It is unusual for us to see this disease. However, stripe rust is a serious threat to our wheat crop. It can do a lot of damage very quickly and should be controlled.

Stripe rust often gives a yellow look to the field. So you may look out across a green field of wheat and see a small area that looks yellow. Looking closer the leaves will have small yellowish rust pustules usually in a line. That is why it's called stripe rust!

The following varieties are the most likely to be infected and are the ones growers should check:

C9436, DG Shirley, NC Cape Fear, NC Neuse, NC Yadkin, P26R12, SS520, SS560, USG3209, USG3592, USG3665, and SS8404

If stripe rust is found it should be sprayed as soon as possible. Two possible products to consider are Folicur (Tebuconazole) and Prosaro. Both products can be applied up to 30 days before harvest.

For more information, click here

For information on rust diseases, click here

-This alert was sent from Dr. Randy Weisz, Department of Crop Science, NCSU

Spot Anthracnose of Dogwood

Spot anthracnose on dogwood in Holly Springs, NC (Wake Co.) on 04 April 2012.
Photo by Mike Munster

Spot anthracnose of dogwood is a perennial problem on one of our loveliest landscape plants. This photo was taken to represent the several questions about this disease we’ve had over the last two weeks. The red to tan spots can be barely noticeable from a distance, but when numerous they can severely stunt and deform the showy "petals", which are actually modified leaves called bracts. Leaf, fruit, and twig infections can also occur. The fungus responsible for the damage is known scientifically as Elsinoë corni. It overwinters in twigs and fruits, and spores infect the bracts in the spring if moisture is present. Fortunately, flowering dogwood is the only plant on which this disease is significant, and the overall health of the tree is not at risk. This disease is very different from “dogwood anthracnose”, caused by the fungus Discula destructiva. In North Carolina this more serious disease occurs only in the mountains.

Susceptibility to spot anthracnose varies among dogwood cultivars. See the table of dogwood cultivars from our Department of Horticultural Science, and their guide to dogwood care. Not much can be done to prevent spot anthracnose, but promoting good air circulation in the vicinity of dogwood trees may be beneficial. Avoid dense plantings or low-lying sites, and prune out dense growth. We generally do not recommend chemical control for tree diseases in the home landscape, where the correct equipment is not generally available. Also, once you see symptoms like those in the photo, it’s too late to get any benefit from a fungicide spray this spring. Looking ahead to next year, a professional applicator may be able to help protect high-value, high-profile trees. Be aware that a protectant spray program may involve spring, summer, and fall applications. Contact your local county cooperative extension service for more information.

Bacterial Leaf Spot: Acidovorax in the Greenhouse

It’s been a busy season for bacterial leaf spots on greenhouse ornamentals. Last week, we blogged about Xanthomonas leaf spot of English ivy. This week, we will take a look at leaf spots caused by Acidovorax sp.

Bacterial spot lesions on hydrangea (Photo: Mike Munster)

Over the past month or so, we have received two petunia samples, a verbena sample, and a geranium sample from 4 different greenhouses across the state. All of these samples had leaf or stem spots. The leaf spots had a water-soaked appearance and plants in advanced stages of infection were collapsing. After examining the spots under a microscope, we concluded a bacterium was the problem.

Water-soaked lesions (Photo: Mike Munster)

Stem lesions (Photo: Mike Munster)

Genera of bacteria that frequently cause leaf spot diseases in floricultural crops include Xanthomonas, Pseudomonas, and Acidovorax. Unlike many fungi, which can be identified by their unique spore types, bacteria are difficult to distinguish based on their appearance. Most bacteria look the same as they ooze out a leaf. It is important to know which species you have so you know whether or not the bacterium present is pathogenic or if it is a secondary saprophyte causing little damage. 

To identify the bacteria responsible for causing the leaf spots in these cases, we had to culture them and then perform additional assays. These assays identify bacteria based on their ability to metabolize different kinds of nutrients and compare the results with a database. Results identified the culprit as an Acidovorax sp. We then took this bacteria and inoculated a healthy leaf to make sure the same symptoms were produced.  

Inoculated leaf (Photo: Mike Munster)

Bacteria belonging to the genus Acidovorax have been isolated from geranium (Pelargonium hortorum) and petunia (Petunia hybrida) since the late 1980’s. The geranium and petunia strains appear very closely related to each other, but different from other strains of Acidovorax, possibly indicating they belong to a new, unidentified species of Acidovorax.

Management recommendations for Acidovorax are similar to those we discussed last week for Xanthomonas. Always start with disease free stock or cuttings. Plants with these symptoms should be removed and destroyed, including the planting mix. Healthy-looking plants adjacent to the diseased plants should be quarantined if possible. Acidovorax is found on petunias, verbena, geranium, hydrangea, and lantana. Greenhouse operators should be sure to inspect all plants in this group if Acidovorax is found.

Leaf spot symptoms on verbena (Photo: Mike Munster)

Leaf spot symptoms on petunia (Photo: Mike Munster)

Keep leaf/stem wetness periods to a minimum once plants come out of propagation, and avoid any kind of physical injury or wounding to the plants, which will aid entry by the bacteria. They can be spread from plant-to-plant by hand, insects, water splash, and even as aerosols. Avoid handling plants when wet, and frequently sanitize knives or other tools used on the plants.

Thoroughly clean and then sanitize pots, flats, surfaces, and tools with steam or chemical disinfectants. There are no effective chemical controls for application directly to the plants, but during propagation copper products can be used to hold back the bacteria somewhat.

Bacterial Leaf Spot on English Ivy

Classic bacterial spot symptoms, notice the yellow halos (Photo: Mike Munster)

We received a sample of English ivy leaves that had necrotic leaf spots of various sizes. The sample came from a greenhouse production system. We viewed the leaves under a microscope and observed bacteria in all of the spots we checked. We also checked for signs of fungal infection since  a fungus in the genus Colletotrichum causes similar symptoms. Typically a yellow halo can be seen around bacterial spots, whereas in the fungal disease, anthracnose, these halos are absent. In this case, the fungus was not observed and bacterial leaf spot was our diagnosis. This is a common disease of English ivy caused by a bacterium in the genus Xanthomonas. While this particular sample was from a greenhouse, this disease is also common in the landscape.  

Bacterial Leaf Spot (Photo: Mike Munster)

Leaf with both bacterial leaf spot and anthracnose.  Notice the yellow halo associated with the bacterial spot (Photo: Mike Munster)

Further tests confirmed that the bacterium Xanthomonas was present in the leaf spots. While we did not identify the bacterium to species, presumably it was Xanthomonas campestris pv. hedera, the common cause of bacterial leaf spot on English ivy (Hedera helix). Hosts include other species of Hedera, Schefflera, and Fatsia. Bacteria are spread by water splash (especially by overhead irrigation) and by handling wet plants. Excessive watering and fertilizer applications promote development of this disease. In a greenhouse environment, the bacteria can potentially come in on cuttings. 

If bacterial leaf spot is diagnosed, adjacent plants probably have latent bacterial infections in the leaves. Pots, flats, and benches are likely to have surface populations of the bacteria. This is especially true if the plants were under a mist system. Symptomatic plants should be discarded, and adjacent ones either discarded or placed in isolation. 

Bacterial Leaf Spot (Photo: Mike Munster)

To control bacterial leaf spot in the greenhouse, be sure to start with clean, inspected cuttings. Different cultivars of English ivy have been shown to have different levels of susceptibility to Xanthomonas. Cultivars with some resistance include, Gold Dust, Perfection, Sweet Heart, Eva, California, and Manda Crested.  Thoroughly clean and then sanitize pots, flats, surfaces, and tools with steam or chemical disinfectants.  There are no effective chemical controls for application directly to the plants, but during propagation copper products can be used to hold back the bacteria somewhat. 

Bacterial Leaf Spot Symptoms (Photo: Mike Munster)

For more information about control of bacterial leaf spot in the greenhouse  click here 

In the landscape, always start with disease free plants and use resistant cultivars when possible. Water plants in the morning to allow the leaves to dry out during the day.  Do not mist plants and avoid getting the leaves overly wet when watering.  Plant plants in areas with low humidity to speed up drying time.  Thin plants at the start of each season to increase ventilation in the beds.  In established beds, remove and destroy any diseased leaves or stems when you first notice symptoms.  Note that English Ivy is an invasive species. It is best used in pots where it can be kept under control. For more information about invasive plant species and planting alternatives click here

Growing Healthy Plants, part I: Sanitation is Not a Dirty Word

With spring nearly upon us, gardeners are dreaming of yards overflowing with healthy flowers, vegetables, and greenery of all kinds. Before you dig your gardening tools out of the shed and start planting, here are some tips for growing happy, healthy plants!

Sanitation is a key to growing healthy, disease-free plants. If you are growing plants in pots, baskets, or raised containers, start with sterile potting media. Old potting media can be full of pathogens ready to attack new plants. Pathogens like Pythium or Phytophthora can survive in soil for years. Using sterile pots goes hand in hand with using sterile soil. Scrub used pots with a stiff brush to remove all soil and organic debris. After a thorough cleaning, sterilize pots by soaking for at least 30 minutes in a solution of 1 part bleach to 9 parts water (about 12 ounces of bleach per gallon).


Dirty equipment is a common means of spreading pathogens to healthy plants. Remember to scrub garden tools and soak them in a bleach solution at the start of the season and after using them to cut or dig around diseased plants. Rinse metal tools after the bleach treatment to avoid corrosion. Alcohol can be used to disinfect cutting tools and is less corrosive than bleach.


Now that you have clean pots, tools, and media, it’s time to pick your plants. Always buy healthy plants from a reliable source: pathogens can enter the garden on diseased plants. A well-run nursery should be very clean, with few weeds, algae, or loose soil on the ground or benches. Nurseries with neat, well-organized displays, minimal crowding, and plants that are neither to wet nor too dry probably use good practices overall and are likely to sell only the healthiest plants. Cull piles and dumpsters should be far away from the sales and production areas. Mimic these practices if you are propagating your own plants.  

Good Practices: Notice the clean benches (Photo: B. Shew)

Poor Practice: Algae, Standing Water, Organic Debris (Photo: E. Lookabaugh)

Poor Practice: Cull pile next to healthy plants (Photo: E. Lookabaugh)

Be sure to inspect all plants before you buy or transplant them. Do not buy or move plants that are wilted, yellowed, stunted, or have any other symptoms or unusual appearance. Inspect the roots of potted plants or transplants. Look for containers or plugs thoroughly colonized (but not pot-bound) with healthy roots. Healthy roots can be brown or white, depending on the species, but they should never have obvious rot or lesions. The potting medium or soil should have a pleasant, earthy odor. Avoid plants growing in stagnant-smelling medium.

Nice White Roots (Photo: E. Lookabaugh)

Be very cautious when dividing or transplanting plants in your yard and be very, very careful with plants from neighbors or plant exchanges. Before putting these plants back into the landscape, brush or wash away excess soil, remove dried leaves, and prune away any dead stems in woody plants. Carefully inspect all parts of the plant for symptoms and do not use any plant that appears to be diseased. If at all possible, pot transplants or divisions in a location well away from your landscape. Watch transplants carefully while they are in your quarantine area and move them to their final destinations only when you are confident that they are disease-free.


In the landscape, rake up old mulch, fallen leaves and other debris before planting and LIGHTLY cover the area with a new layer of mulch after planting. Once you are done planting, be sure to monitor plants closely. Promptly remove all fallen leaves, spent flowers, unwanted produce, and dead or cankered stems. In many cases, composting cannot be counted on to kill pathogens, so promptly destroy any materials you removed from diseased plants or their surroundings.

Remove fallen, diseased flowers to prevent inoculum splashing up (Photo: B. Shew)

If you start seeing disease symptoms, contact your local county agent or the Plant Disease and Insect Clinic. Sometimes, our diagnosticians can diagnosis plant disorders from digital images. Take a photo and email it to us. We will be able to advise you as to whether you should remove the plant, or if sending a physical sample is necessary to diagnose the problem.

Sometimes gardening is best done with a buddy!

For more information on sample submission see our webpage: 
http://www.cals.ncsu.edu/plantpath/extension/clinic/submit-sample.html