Super Sad Sunflowers: Alternaria leaf and stem spot

Field of sunflowers (Photo: B.B.Shew)

Sunflowers are becoming an increasingly popular alternative crop in North Carolina. A field of sunflowers in bloom is beautiful, but sunflowers are most important as an oil crop. Sunflower oil is lighter in taste and supplies more Vitamin E than other vegetable oils. Sunflower oil is particularly appealing to the food industry because it is trans-fat free and stable without hydrogenation, making it excellent for frying and increasing shelf life. Since most sunflowers are produced in the Great Plains region of the U.S., little is known about which diseases and insect pests may become problematic or limit sunflower production in North Carolina.

As the growing season came to an end, we received several sunflower samples that we diagnosed with Alternaria leaf and stem spot. In the traditional growing areas of the Great Plains, Alternaria is usually of minor importance because conditions are not favorable to severe disease development. Unfortunately for us, North Carolina has hot and humid summers perfectly suited for Alternaria disease development.

Alternaria disease symptoms (Photo: E.C. Lookabaugh)

Alternaria leaf and stem spot is caused by the fungus Alternaria. Two species of Alternaria cause disease on sunflowers, Alternaria helianthi (now known as Alterniaster helianthi) and Alternaria zinniae with A. helianthi being more common. Symptoms include irregular leaf spots, stem lesions, and dark brown spots on the seed heads. Leaf spots are dark brown with grey centers. Yellow halos around spots are seen on younger foliage. The stem lesions begin as dark flecks that enlarge to form large blackened narrow lesions. Severe infections result in defoliation and stem lodging.

Flower head symptoms (Photo: E.C. Lookabaugh)

Stem lesions (Photo: E.C. Lookabaugh)

The pathogen survives on infested plant debris and on the alternate hosts safflower and cocklebur. Cocklebur is native to the U.S and is found throughout NC. Safflower is not present in NC. The fungus is spread by wind and splashing water. Spores splash from infested crop debris onto the lower leaves and stems, where symptoms first appear. Warm, humid conditions favor disease development and spread.

Control of this disease can be achieved through a strict sanitation program, crop rotation, and the use of preventative fungicides. Sanitation is key. Disease plants should be removed and destroyed. Cultural methods that limit splash dispersal and long periods of leaf wetness probably will help to reduce disease. These include avoiding overhead irrigation and using proper plant spacing to promote air circulation. This disease can be seedborne. Use certified, disease-free seed that is produced in dry regions where Alternaria is not a problem. Do not save seed from diseased plants. Do not grow sunflowers two years in a row in the same field because the fungus overwinters on plant debris. Tillage operations promote debris decomposition and reduce the chances of splash dispersal of the fungus. Preventative fungicide applications can be started when symptoms first appear or at flowering, since this is when plants become most susceptible to the disease.

Happy Sunflower (Photo: B.B. Shew)

Post by Emma Lookabaugh and Barbara Shew

Southern Stem Rot on Peanut

Written by Barbara Shew 

The fungus Sclerotium rolfsii has over one thousand plant hosts and causes diseases on field, vegetable, and ornamental crops grown in North Carolina. On many hosts, the first sign of infection are profuse strands of white, fan-shaped fungal growth at the base of the plant. Later, the fungus produces large numbers of round, tan-to-brown sclerotia that resemble mustard seed.

Stem Rot on Peanut ( Photo: B. Shew)

On peanut, the disease caused by S. rolfsii is known as southern stem rot, stem rot, or white mold. White fungus growth appears when the environment is right, often just following a rain (Figure 1). However, southern stem rot can be very damaging on peanut even when the distinctive fungus is not apparent; the damage often is hidden below ground. In addition, above-ground symptoms of stem rot can be hard to distinguish from other important peanut diseases, including CBR, Sclerotinia blight, and Rhizoctonia limb rot. The best way to determine the severity of a stem rot problem is to check the plants within an hour or two of digging. It is a good idea to check peanuts for symptoms of stem rot, CBR, and nematode damage immediately after digging. Growers should note problem areas so that appropriate controls can be used the next time peanuts are grown. I prepared a short video pointing out the distinctive features of stem rot on freshly dug peanut plants.



For more information about peanut diseases and their control, check out these links:
http://www.cals.ncsu.edu/plantpath/extension/clinic/fact_sheets/index.php?do=plant&id=1
http://www.peanut.ncsu.edu/

Box Blight Variety Trials

Box blight is a new disease to the U.S. and is caused by the fungus, Cylindrocladium buxicola.  Box blight can seriously affect the appearance and aesthetic value of boxwoods. Typical symptoms include circular leaf spots with dark brown to purple margins, dark brown to black stem lesions, blighted foliage, and eventual leaf drop.  Under humid conditions,  white fuzzy spore masses can be found on infected stems and leaves.  Disease develops rapidly in warm, humid conditions and is more severe in shade conditions.  Boxwood varieties differ in their susceptibility to box blight.  A recent study determined that B. sempervirens types are more susceptible to box blight.  B. microphylla var. japonica 'Green Beauty', B. sinica var. insularis 'Nana', B. harlandii, and B. microphylla 'Golden Dream were all found to have some level of resistance to box blight.  Be sure to remember that some boxwood varieties are limited in their optimal plant hardiness zones.  

Box Blight Symptoms

For more information on Box Blight, click here.  For more information on the boxwood variety trials, click here.  

Honey Mushrooms Galore

Last week NC State hosted the 18th Ornamental Workshop on Diseases and Insects at the Kanuga Conference Center in Hendersonville, NC.  Ornamental pathologists, entomologists, and diagnosticians from across the country spent the week discussing all things related to ornamental diseases and insect pests.  Lucky for us, we were also able to carve out some time to do a little hiking/ mushroom hunting in Pisgah National Forest.  Over the course of our 5 mile hike, we saw one black snake, one timber rattlesnake, and thousands of honey mushrooms! Apparently Pisgah National Forest is a hotbed of Armillaria.  With all of the recent rain, you should start seeing Armillaria honey mushrooms popping up in your neck of the woods as well.  For more information on Armillaria root rot, see our previous blog post here.

Honey Mushrooms (Photo: E.C. Lookabaugh)

Asiatic Soybean Rust Update

Reprinted from North Carolina Pest News
Steve Koenning, Extension Plant Pathologist
Jim Dunphy, Extension Soybean Specialist

Asiatic soybean rust has been confirmed in Robeson and Johnston counties, North Carolina. Between this find, and the confirmation of rust in Robeson County, North Carolina, and in Union County, Georgia, this puts rust approximately 105 miles from Charlotte, 140 miles from Elizabeth City, 15 miles from Fayetteville, 15 miles from Murphy, 35 miles from Raleigh, 80 miles from Washington, 75 miles from Wilmington, and 80 miles from Winston-Salem, North Carolina.

We do not recommend spraying soybeans with a fungicide to control Asiatic soybean rust if they are not yet blooming, if they are blooming but rust has not been confirmed within 100 miles, or if full sized seeds are present in the top of the plant (stage R6). Such pre-bloom applications have seldom improved yields, and repeated applications will likely be needed to provide season-long protection against rust. The higher labeled rates tend to provide more days of prevention, and may thus require fewer applications. The triazole fungicides, alone or in combination with a strobilurin fungicide, will probably provide better prevention of rust than a strobilurin alone. Be sure to check the fungicide label to see how many times it may be used in a season.

An exception to the above recommendation is if Asiatic soybean rust is found on the farm before bloom, spraying a fungicide to the rest of the fields on the farm is recommended. Soybeans that have just reached full bloom (stage R2) typically have 65 days until they’re safe from rust or frost (stage R7) if they are full-season soybeans, or closer to 55 days if they are double-crop soybeans. If they have small pods in the top of the plants (stage R3), they have 55 and 47 days, respectively, to R7. With full sized pods in the top of the plants (stage R4), they have 45 and 38 days, respectively, until R7. From stage R5 (small seeds in the top of the plant) they typically have 35 and 30 days, respectively. From stage R6 (full sized seeds in the top of the plants), they typically have 20 and 17 days, respectively.

Rust will typically take 10 to 20 days from initial infection to develop to detectable levels. It will take another 7 to 14 days to spread to other leaves on the same plant, and another 10 days to cause significant defoliation.” This assumes optimal conditions for rust, “65 to 85 degrees, and either overcast or rainfall” through much of this period. This is not common in North Carolina in September and October but has and can occur.

The rust prediction models say there was a fair to good chance rust spores were deposited in North Carolina this weekend. If so, we expect to detect rust in about three weeks in sentinel plots, which would be about October 1. It will likely take another two weeks with optimal conditions for rust to increase to damaging levels.

The current status of soybean rust in the U.S. can always be found at http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi.

Need Weekend Plans? Exciting Happenings in the Triangle

Looking for something exciting to do on Saturday? If so, join NC State's Department of Plant Pathology in two fun-filled activities! On Saturday, the Plant Pathology Graduate Student Association (in conjunction with the Plant Pathology Society of North Carolina and USDA APHIS PPQ Center for Plant Health Science and Technology) will be hosting the Bug Bus exhibit at BugFest.  BugFest is an annual event put on by the North Carolina Museum of Natural Sciences.  BugFest features displays, exhibits and tons of activities for children both inside and outside of the museum. Every year BugFest has a different theme insect with this years being mantids! Need to brush up on your mantid knowledge before the big day? Then check out this great post from the Museum of Natural Sciences blog, Mantid Madness.  Our exhibit, the Bug Bus, features insect vectors of plant pathogens.  Kids can hop on the Bug Bus and collect stamps in their BugFest passports as they learn about different plant diseases and pests.  Our booth will be located inside the museum on the first floor (underneath the whale). So, if you are brave enough to make your way through hoards of screaming children, then come check us out! It's FREE! 

More information on BugFest can be found here.

Well if BugFest doesn't whet your appetite, maybe football and barbecue will.  NC State's Department of Plant Pathology will also have a booth at the annual CALS Tailgate.  Tailgate is the largest single Alumni event held at NC State University.  Tailgate "is a showcase for the College's academic, research and extension mission, packed around fun events like a silent auction, live band, departmental displays, children's games, great food, fun and friendship!" McCall's will be catering an all you can eat BBQ buffet.  Join us as we cheer on the Wolfpack! 

More information on CALS Tailgate can be found here.  

Sample of the Week: Poinsettia scab

Rapid elongation of poinsettia stem infected
with Sphaceloma poinsettiae

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


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

Close-up of poinsettia stem showing scab lesions

Scab lesions on poinsettia leaf, caused by Sphaceloma poinsettiae

Don't Forget Plants, Pests, and Pathogens: August 28th 2012

Plants, Pests and Pathogens is an in-service training program for Horticulture Extension Agents and Extension Master Gardeners.

The Plants, Pests, and Pathogen sessions will be conducted on the fourth Tuesday of February, April, June, August, and October 2012 from 10:00 am to 12:00 pm. Every session includes a discussion of current plant disease and insect problems by the Plant Disease and Insect Clinic's Mike Munster and Dave Stephan.

Find out about the many other exciting presentations planned for this year by checking out the Master Gardener Plants Pests and pathogens webpage. We hope to see you on-line!

Log On To August meeting: click here

General information: click here


Info for Master Gardeners: click here

 
Info for agents: click here