Thursday, May 19, 2011

First Slime Flux Sample of the Year Arrives in Clinic

Slime flux on white oak
(Photo by: Randy Cyr, Bugwood)
The first slime flux sample of the season was sent to the clinic today, as a digital image from a Raleigh homeowner.  This is a problem we usually associate with the warmest months of the year. Slime flux is a foul-smelling and unsightly seepage of sap from the trunk of shade trees.  Slime flux occurs in apple, birch, elm, hemlock, maple, mulberry, oak, poplar, and willow.  In North Carolina, slime flux is quite common in the landscape on large, mature oaks, tulip poplars, and elms. In oaks, the fluxing usually occurs on the lower portion of the trunk, close to the ground.

The underlying cause of slime flux is a condition called wet wood, which develops when bacteria ferment the sap within the wood. These bacteria enter through wounds, usually in the roots. The affected wood turns dark and appears water soaked.  As the sap is fermented, methane and carbon dioxide gases can be produced. If the internal pressure from the gas becomes great enough, the fermented sap seeps out of cracks or wounds and down the bark. The flux is colorless to tan at first, but darkens upon exposure to the air.  As fluxing continues, large areas of the bark can become soaked. 

Many different microorganisms grow in the flux, which results in a foul or alcoholic smell.  Various types of insects, including bees, wasps and butterflies, are commonly attracted to it.

Some dieback may occur in severely affected trees, but often the fluxing stops after several weeks or months with no apparent damage to the tree. The slime flux may be triggered by heat, drought, or other stresses, so try to maintain good growing conditions.  There is no curative treatment for slime flux, but it will do no harm to remove loose bark over the area. Wet wood and slime flux are one more reason to avoid causing wounds to tree roots.

For more info: Click Here

Tuesday, May 17, 2011

My plants have a little sugar coating :) Powdery Mildew Spotted in the Landscape

Powdery Mildew on Crape Myrtle
(Photo: H.D. Shew)
Powdery mildew has started to show up on many landscape plants. Powdery mildew is easily diagnosed on most plants by the characteristic coating on leaves that gives this disease its name. The “powder” will not rub off – it is actually the mycelium and spores of the disease-causing fungus. 

Powdery mildews frequently infect dogwoods, crape myrtles, roses, lilacs, tulip-trees, oaks, and numerous other shrubs and perennial plants.  In addition to the powdered appearance, typical symptoms include discolored and distorted leaves, shoots, and flowers. Although powdery mildews rarely cause serious damage to most hosts, severe cases can result in defoliation or poor growth. 

PM on fruit
(Photo: H.D. Shew)
PM on flower stem
(Photo: H.D. Shew)
While most powdery mildews look the same to the naked eye, hundreds of different species of fungi cause powdery mildews. Most are specific to a particular host. Under the microscope, different groups of powdery mildews can be identified by their highly distinctive cleistothecia (specialized spore-bearing structures).
PM micrograph
Note hooked appendages
(Photo: H.D. Shew)
PM micrograph
Note branched appendages
(Photo: H.D. Shew)

In the spring, cleistothecia from previous infections rupture and release wind-blown spores.  These spores land on susceptible plant tissue and begin to grow on the surface and into the upper layer on the leaf.   The fungus produces columns of asexual spores that are capable of causing new cycles of infection.  Late in the season, the cleistothecia appear as tiny dark specks embedded in the mildew.  These contain the sexual spores of the fungus and will overwinter to start new infections in the spring.
Black cleistothecia embedded in mycelial mat
on leaf surface (Photo: H.D. Shew)
Powdery mildew is most common in cool or warm and humid weather conditions, but it may occur and cause severe damage in warm and dry climates. Temperatures between 70-80 degrees F and a relative humidity between 85%-100% favor infection. Powdery mildew pathogens are some of the only fungal organisms that can germinate and infect in the absence of free water.  These fungi require high humidity but are not dependent on wet leaves for infection and spread.  Excess leaf wetness actually inhibits spore germination. 

Cultural Control:
  • Purchase mildew resistant varieties
  • Reduce humidity: Increase air circulation and light penetration by thinning and pruning plants to reduce overcrowding
  • Avoid planting susceptible plants in shaded areas
  • Sanitation: Prune out dead and diseased tissue and rake and remove any fallen tissue to reduce the chances of the fungus surviving until the following season
  • Overhead watering of the leaves: Water inhibits spore germination for most powdery mildews, but be careful because excess leaf wetness can increase chances of other foliar diseases.  
  • Avoid practices that stimulate succulent growth: Powdery mildews are obligate parasites that prefer fresh, succulent plant growth. Applying nitrogen fertilizer, pruning heavily, and watering excessively are not recommended because they promote succulent growth.  
Powdery mildew on dogwood (Photo: H.D. Shew)
Chemical Control: 
Chemical control is often not necessary in the landscape because this disease rarely kills the plant.  Spraying trees such as dogwoods and crape myrtles can be impractical. 

Fungicides may be used to control powdery mildew on highly susceptible varieties of roses and other plants that suffer severe symptoms and damage.

See Rose Diseases and Their Control in the Home Garden
Rose Diseases Link

Tuesday, May 10, 2011

Sweet Sounds of Springtime

Periodical Cicada. Photo by Susan Ellis Bugwood
One of my favorite things about summertime in the south is the humming and buzzing of cicadas.  The distinctive buzzing noise that fills the air across the south is the male’s mating call.   Right now, brood XIX of the periodical cicadas are waking up and preparing to swarm.  Brood XIX are periodical cicadas, meaning that all members of the brood emerge in the same year.  Brood XIX, also known as the Great Southern Brood, has been living underground since 1998.  This is the country’s largest brood, stretching across 12 states.

Every summer, the dog-day cicadas (Tibicen sp.) come out and fill the air with their mating calls.  These cicadas are larger than the periodical cicadas and have green markings.  The periodical cicadas are smaller, with hints of orange or red to their bodies.  This year, we will have the opportunity to see and hear both types of cicadas.  Periodical cicadas sing and fly in spring, whereas other species of cicadas are active during the summer.  

Cast "skins" and cicadas that were
unsuccessful emerging at base of tree.
Jordan Lake area. Photo by Steve Bambara
Both types of cicadas have a life cycle that takes them from tree to soil and back to tree again.  The life cycle of the Brood XIX periodical cicada takes 13 years to complete, while the speedy dog-day cicadas take only 2-5 years and are not synchronized with each other.  Several other broods of periodic cicadas emerge only once every 17 years!

After spending 2-17 years in the soil, cicada nymphs dig their way to the surface.  In late May or early June, the nymphs crawl to the trunk of a tree or some other tall object, and cling there.  Soon the insect molts into the winged adult stage, leaving behind the cast skin.  The shells left behind are fun to play with and also give back nutrients to the tree as they decay.

Cast skins clinging to sweet gum branch. Jordan Lake area
May 9, 2011.
Photo by Steve Bambara
Adults are active during the daylight hours.  Males begin to sing with a shrill buzzing noise to attract the females.  After mating, females used their sawlike ovipositors to split open the bark of hardwood twigs and insert their eggs.  After 6 or 7 weeks, the eggs hatch and tiny ant-like first stage nymphs drop to the soil to burrow for the next 2 or more years.  While in the soil, the nymphs feed on the roots of trees.
Split twigs showing egg deposits. Photo NCSU 

Aside from their daytime racket, the cicadas are otherwise harmless.  The only damage from cicadas occurs when they split tree twigs to lay their eggs.  This causes tip dieback (or natural pruning!), which is only a problem on a tiny tree. 

It is neither practical nor desirable to try and eliminate cicadas! They provide an abundant food source for local predators… a tasty buffet for birds.

Watch this neat video from Charlotte NC and hear the cicadas in action!

For more information: click here
Special Thanks to Stephen Bambara, NCSU Entomology for information and photos!