Wednesday, August 31, 2011

Peanut Disease Control Following Hurricane Irene

Written by: Dr. Barbara Shew, Peanut Extension Specialist

Sclerotinia blight
Heavy rain from Irene, wet vines, and the recent cool nights make ideal conditions for Sclerotinia blight. Today (Aug 31), all reporting weather stations in North Carolina are advising sprays and indicating high to very risk of Sclerotinia blight. Conditions will remain highly favorable for Sclerotinia blight for at least the next 5-7 days.
Make sure that fields with a history of Sclerotinia blight are protected. The last effective spray date for today is August 10. This accounts for 3 weeks of protection following a fungicide application. Fields that have not been sprayed since August 10 are not protected.  

It is very important to catch Sclerotinia outbreaks when they first occur – this is the time when fungicides are most effective at controlling the disease.  Scout carefully if you are in doubt about your field history or control efforts to date. 

Even in fairly advanced cases, Sclerotinia blight can be hard to see unless you check plants thoroughly.  Scout by checking 50 feet of row in several locations across a field.  Part the rows and check inside the canopy for the fluffy growth of the Sclerotinia blight fungus on stems, leaves, and pegs. Infections may be present on leaves and stems that are not touching the ground, particularly after a heavy rain. Other signs and symptoms include bleached and shredded stems and black fungus structures (sclerotia) that look like mouse or insect droppings. Sclerotia often are found on or inside stems and pods.
Early development of Sclerotinia on stem
Stem shredding by Sclerotinia (Photo by Bridget Lassiter)

We are currently are working on yield loss models for Sclerotinia blight. While we have not finished our analysis, we have a rough idea of what to expect.  Assuming a Sclerotinia-free yield of 4,600 lb/a, yield decreases about 50 lb/a for every 1% incidence (plants diseased) of Sclerotinia blight at 110 days after planting. This suggests that a reasonable treatment threshold at this point in the season is between 2-3% of plants diseased. If you expect a delayed harvest, I would definitely lean to the low end of this range (or even lower) so that you can maintain good control through the next several weeks. 

Leaf spots and web blotch
Late leaf spot thrives when leaves are wet and night temperatures drop a bit. Late leaf spot is more difficult to control than early leaf spot. It will be very important to stay on top of late leaf spot control for the next couple of weeks - or longer if you expect a delayed harvest.  Be particularly watchful on highly susceptible cultivars like Gregory and Perry.

We have seen little to no web blotch in the past several years, but outbreaks sometimes follow a tropical storm. Web blotch is recognized as large (½-inch) dark patches or blotches with faint or irregular margins. They are found only on the upper surface of the leaf at first. Young lesions have a grayish cast but later the blotches turn light brown.  NC-V11 and VA 98R are highly susceptible to web blotch and most prone to outbreaks. A good leaf spot control program usually will control web blotch.
Web blotch

Headline will give excellent protection against late leaf spot and web blotch at 9 -12 oz/a. Most other foliar fungicides also perform well against these diseases. Avoid using tebuconazole since it is very weak against late leaf spot. Use a multi-site fungicide such as Bravo (chlorothalonil) for the last spray of the season. This will help to reduce the risk of developing pathogen populations that are resistant to other fungicides. 

Peanut rust is rare in North Carolina, but sometimes shows up after a storm. Peanut rust is NOT the same rust that infects soybeans, but in some ways it is similar. Like soybean rust, peanut rust does not survive our winters. It is a sporadic problem in the southern-most peanut production areas of the US and is common in Central American and Caribbean countries. As with soybean rust, spores of peanut rust can be transported over long distances by hurricanes and other storms. Considering the path that Hurricane Irene took, it is possible that we will see scattered outbreaks of peanut rust.

Peanut rust produces numerous small reddish-brown pustules on the undersurface of the leaf. The pustules erupt to release millions of rust-red spores. These spores will stain a white cloth or paper that is swiped on the underside of a rusted leaf.  Rust epidemics start in hot spots and spread quickly. Fungicides that are effective against rust include Bravo, Abound, and tebuconazole. Tilt (propiconazole) is not effective.

Friday, August 26, 2011

Corn Smut: Friend or Foe?

Written by: Dr. Barbara Shew and Emma Lookabaugh

Sweet corn at the local farmers market
(Photo by: H.D. Shew)

On summer days, farmer’s markets are bursting with sweet corn. Shoppers buy ears by the dozens for good eating now and to stock their freezers for the winter. Corn – both field and sweet – is a relatively healthy crop and diseases usually do not trouble the home gardener. An exception is corn smut – it’s both common and easily recognized by its grotesque tumor-like galls and masses of dark sooty spores.   
Recently, Dr. Mike Benson, a professor in our Department, brought in some smutted ears of sweet corn for class.  The infected super sweet corn was his second planting during the first week of June.  He noted that the fungus didn’t start showing up until the ears starting filling in during the first week of August and estimated that about 15% of his plants were infected in mid-August. 

The corn smut fungus, Ustilago maydis, attacks corn (or maize; Zea mays) and its wild relatives (Zea spp.).  It can infect all actively growing corn tissues, but the most obvious symptoms are tumor-like galls on the ears.  Young galls are white and firm and are covered with a semi-glossy periderm, which eventually ruptures, exposing masses of sooty teliospores.  
Young corn smut galls
(Photo: H.D. Shew)
The teliospores serve as overwintering inoculum for future corn crops.  In the spring, teliospores are windblown or rain-splashed to nearby corn plants. The teliospores germinate to form sexual spores called basidiospores or sporidia.  The sporidia can also be dispersed in wind or rain. Later, two sporidia fuse and give rise to the infective stage of the fungus.  

Any above-ground plant part can be infected, including ears, tassels, silks, stalks, nodal shoots and leaf midribs. Distorted tissue may be noticeable within days after infection. Typically, galls form within a week and continue enlarging for up to three additional weeks.  Initially, galls are composed primarily of host tissue undergoing tumor-like growth. As the galls age, their fleshy interior becomes streaked with black as dark sooty teliospores begin to replace the white host tissue. Eventually the gall becomes a mass of spores, dehydrates, and ruptures, releasing the teliospores.  
Mature corn smut galls
(Photo: H.D. Shew)
Individual developing kernels are infected through the silks. Kernels are not susceptible to infection once pollination occurs, so any condition that reduces pollination favors smut infections. Drought stress (when pollen dies or tassels stop producing pollen) or extremely wet, humid weather (causing poor or no pollen production) opens up the window for infection.  University of Illinois Professor Emeritus Jerald Pataky says that “wimpy males,” that is, plants that don’t produce pollen in adverse conditions or males (tassels) that are not ready when the females (ears) are ready for pollination, are more susceptible to smut.  
Gall on single kernel
(Photo: H.D. Shew)
Smut usually makes sweet corn unmarketable due to cosmetic damage, but in parts of Mexico, it is actually prized. Traditional farmers gather young galls after natural infection and market them as Cuitlacoche (or Huitlacoche), a delicacy in Mexican cuisine.  Often called the “Mexican truffle” or a “food of the gods,” it has been served up in Mexico since Aztec times.  Nowadays, there is great interest in developing techniques for inoculating ears with smut and marketing the galls as gourmet fungi to upscale restaurants. It is a favorite ingredient in many soups, appetizers, and entrees because of its unique combination of earthy and corn flavors.  Even here in Raleigh, NC, you can order up cuitlacoche and get a taste of this traditional Mexican delicacy. 
Chicken breast stuffed with smut fungus
(Photo: B.B. Shew, Jibarra Restaurant)
In case you *don't* want smut in your corn, control options are limited. Rotation is somewhat helpful and it may help to remove and destroy (or eat!) smutted ears before the galls erupt.  “Resistant” varieties are commonly recommended for control, but performance can be erratic since most are not truly resistant but instead escape infection. Choose varieties that are known to perform well in your region. Make several plantings during the spring. If all goes well, this will keep you in a steady supply of sweet corn for the summer. This also increases the chances that some of your plantings will escape major problems with poor pollination and smut.  
Corn at the farmers market
(Photo: H.D.Shew)

References and links of interest:

Pataky, J. K., and K. M. Snetselaar. 2006. Common smut of corn. The Plant Health Instructor. DOI:10.1094/PHI-I-2006-0927-01

Jerald Pataky’s cuitlacoche recipes page

In Mexico, Tar-Like Fungus Considered Delicacy

Jibarra Modern Mexican Restaurant, Raleigh, NC

Mary Ann Hansen. Corn Smut. Virginia cooperative extension publication 450-706.

Special thanks to Dr. Pataky for help with this blog!

Wednesday, August 24, 2011

Armillaria Root Rot in the Landscape: Attack of the "Humongous Fungus"

Armillaria Fruiting Bodies (NCSU PP Slide Collection)
Root rot is one of the most commonly diagnosed disease problems of woody plants in landscapes in North Carolina.  
Each year we see dozens of shrubs and trees that have one of the “big three” root rotting diseases: Phytophthora root rot, Thielaviopsis black root rot, or Armillaria root rot.  

This post will feature Armillaria root rot.  Be on the lookout in the upcoming weeks for posts on Phytophthora root rot and
Thielaviopsis black root rot!

Armillaria root rot is caused by species of the fungus Armillaria.  Common names for this group of fungi include the oak fungi, shoestring root rot fungi, or the honey fungi, the latter referring to the honey-colored mushrooms the fungus produces.  

Armillaria is a common soil inhabitant and can infect a very wide host range including; oaks, maples, azaleas, beeches, birches, boxwoods, cedars, dogwoods, firs, poplars, rhododendrons, yews, roses, spruces, and sycamores (pretty much any woody tree or shrub).  It can be destructive in orchards or on fruit trees in the landscape. 
Symptoms of Armillaria Root Rot (NCSU PDIC) 

Armillaria is typically a problem in older plants or plants that have been stressed due to drought, frost, insect attack, mechanical injuries, poor drainage, low soil fertility excessive shade, or pollution damage.  However, it can be an aggressive pathogen under some conditions.  Severe infections of young plants in nurseries can result in a quick death.  Older or mature plants can withstand infections for several years, resulting in a slow decline, eventually ending with the death of the plant.  

Above-ground symptoms include leaf drop, dieback, and an overall decline in plant vigor.  On conifers, the crowns of infected plants start to thin and change colors, often turning red, brown, or yellowish. Conifer infections sometimes result in heavy resin flow at the tree base.  
Armillaria Symptoms on Juniper (NCSU PDIC)

Armillaria infections start in young roots, but soon the fungus begins to decay larger woody roots. In the early stages of infection, wood becomes slightly grey and can have a water-soaked appearance.  Later, the wood becomes light yellow to white and has a soft, spongy rot.  Often, rotted areas are offset by black lines of zonation.  
Note the black lines of zonation (Photo: Bugwood)

Severely diseased trees can pose significant safety hazards during storms since branches and bases are weakened and can easily break under windy conditions or if under added pressure from snow or ice.  
Windthrow Hazard (Photo: Bugwood)

Unlike most plant pathogenic fungi, Armillaria produces mushrooms and other structures that are visible to the naked eye. Three diagnostic signs of Armillaria root rot include:

1.  Mycelial Fans: The most common diagnostic sign of this disease can be found beneath the bark (between the bark and the wood) at the base of the tree or shrub.  White or creamy paper-like mycelial fans can be observed when the outer bark is carefully peeled away.   These white mycelial fans can also be found beneath the bark of infected roots and root collar area. 
White Mycelial Fans (Photo: Bugwood)
White Mycelium Under Bark (E. Lookabaugh)

2.  Black Rhizomorphs: Sometimes rhizomorphs (dense strings of mycelium) that look like black shoestrings can be found under the bark or throughout the soil around infected tissue.  Rhizomorphs serve as one of the primary means of dissemination.  Rhizomorphs grow through the soil from infected trees, roots, or old stumps.  They are able to directly penetrate healthy roots and cause disease. 
Shoestring-like Rhizomorphs (Photo: Bugwood)

3.  Honey Mushrooms:  In the fall, honey-colored mushrooms can be seen growing near the base of diseased trees and shrubs.  Typically, these mushrooms grow in clusters.  These mushroom produce microscopic basidiospores, but the spores are not thought to play an important role in the spread of the disease. Most species of Armillaria are edible and are quite tasty.  Caution: It is always best to have any mushrooms identified by an expert before you eat them.  Eating misidentified mushrooms can be fatal! 
Honey Mushrooms (NCSU PP Slide Collection)


Usually homeowners do not notice Armillaria root rot until the plant is dead or dying. No control is possible at this point and the plant should be removed. 

Replanting can be problematic because Armillaria can survive for many years as rhizomorphs in soil or in old wood and stumps. Remove the affected plant and thoroughly dig up and remove all large roots, stumps and any other wood or prunings from the affected area. When planting in areas where a plant has died, or where trees have been removed, as in new construction, remove all old roots, stumps, and wood before replanting.  Consider planting ornamental herbaceous or perennial plants or grasses in the area for a few years before attempting to replant woody species.

Healthy trees and shrubs are better able to resist Armillaria root rot than stressed plants. Choose species that are well-adapted to your region and growing site. Maintain their health by fertilizing as recommended, watering during dry spells, and improving drainage in wet areas. When possible, prevent defoliation from insects and foliar diseases. Be careful to avoid damage to roots when digging or tilling. Do not push up soil around tree trunks and do not move soil from affected areas into sites where woody species are growing. 

Fun Facts:

This fungus glows in the dark! The mushrooms themselves do not glow much, mainly just the mycelium, giving infected/exposed wood an eerie glow at night.  Cutting open a piece of wood with advance decay gives you the greatest chance of seeing the luminescence.  Traditionally, glowing wood has been featured in folklore and mythology and termed “fairy fire” or “foxfire.” We do not know the exact reason why the mycelium glows, but we still think it makes this fungus pretty cool!
Glowing Mycelium (Photo: John Denk)

Back in the 1990’s, an article came out highlighting the ability of Armillaria bulbosa to form very large clones.  The ability of Armillaria to produce rhizomorphs, allows this fungus to spread out and become quite large, covering great geographic distances.  As a result, the phrase “humongous fungus” was born and the media took hold.
UHAUL "Humongous Fungus" (Photo: Tom Volk Website)
 Special Thanks to Dr. Larry Grand for Helping with this Post!

Monday, August 22, 2011

Plants Pests and Pathogens

We hope you can join us for Plants, Pests and Pathogens, Tuesday 8/23/2011 at 10am.

You can participate in this two-hour training opportunity for Master Gardeners and Horticulture Agents via your computer, or you can go to one the of downlink sites. To participate from your computer:
  • Log In by 9:30 am. Include your county with your name and leave the password field blank
  • Take a few minutes ahead of time to visit the page on How to Use Elluminate. Check to be sure that you have downloaded the free software required to run the program.  There are one page reference guides and tutorials available.

In order to minimize interference, please keep your microphone turned off, except when you are speaking to the group.

Schedule for August 23 Session:

  • 10:05 am- Regional Update, Coastal Plains: Cyndi Lauderdale, Wilson
  • 10:10 am-Regional Update, Piedmont: Mary Helen Ferguson, Randolph
  • 10:15 am- Regional Update, Mountain/Foothills: John Vining, Polk County
  • 10:20 am- Featured Speaker: Frank Louws, Organic Disease Management for Vegetable Gardens
  • 10:58 am- Show Stopper Plants: Climbing Hydrangea, John Vining
  • 11:00 am- Entomology: David Stephan, NCSU PDIC, Insect Identification Specialist
  • 11:35 am- Current Issues in Plant Pathology: Mike Munster, PDIC