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)