Thursday, September 29, 2011

Sooty Blotch and Flyspeck of Apples

Sooty Blotch and Flyspeck (Photo: Bugwood)
With fall coming fast, it’s time to gear up for apple season. Before you make your trip to the local farmer’s market, we decided to introduce you to the two most common apple diseases in North Carolina, sooty blotch and flyspeck. Without the use of fungicides, these two diseases would affect virtually all apples grown in the southeastern United States. These diseases are found on all cultivars of apples and both diseases can be found on the same apple at the same time.

The most common symptom associated with sooty blotch is the growth of feathery, olive green fungal colonies on the surface of mature fruit. Flyspeck can be recognized by the presence of very small, shiny, black, dots arranged in an irregular or circular pattern on the fruit surface. This gives fruit the unappetizing appearance of being covered with fly droppings.

Sooty Blotch and Flyspeck  (Photo: Bugwood)  
Both of these diseases cause only superficial damage on the apple surface. The fungi that cause them, grow on the surface of the cuticle and do not damage the apple itself or affect its flavor or quality. Sooty blotch and flyspeck cause losses in commercial apple production because affected fruit are downgraded from valuable fresh market grades to much cheaper processing or juice grades.
Sooty Blotch and Flyspeck (Photo: PDIC Database)

Over 60 putative species of fungi have been associated with sooty blotch and flyspeck worldwide. In the Southeast, Peltaster fructicola, Leptodontidum elaitus, Stomiopeltis spp. and Geastrumia polystigmatis are the most common species associated with sooty blotch and Schizothyrium pomi is the most common species associated with flyspeck. Fungi that cause sooty blotch survive the winter on apple twigs and reservoir hosts surrounding the orchard. Spores are dispersed by wind and windblown rainwater to developing fruit in the spring and early summer. Secondary spread occurs throughout the summer. Usually, symptoms can be seen 20 to 25 days after infection.

S. pomi also overwinters on apple twigs and other perennial reservoir hosts. Airborne ascospores are the primary means of infection and are usually produced for about 2 months beginning around bloom. Specks, which are actually fruiting bodies of the fungus, appear about 3 to 6 weeks after infection. Secondary spread occurs through windblown conidia produced on infected fruit and twigs and on reservoir hosts surrounding the orchard.

Sooty Blotch (Photo: PDIC Database)
Commercial growers control these diseases with cultural practices and fungicide applications. Cultural control practices include pruning during dormant and summer seasons and fruit thinning. Pruning and fruit thinning reduce the drying time within the canopy and allow better fungicide penetration through the canopy. Sooty blotch and flyspeck fungi require wetness to infect. Pruning and fruit thinning discourages these fungi by increasing airflow through the canopy. Mowing weeds and grasses under apple trees also helps trees stay dry. Another important cultural practice is removal of surrounding reservoir hosts, especially brambles. Surrounding blackberry plants act as harbors where these fungi are able to multiply during the season and then spread to nearby apple fruit.

In addition to cultural practices, these diseases are controlled with preventative fungicide applications. Preventative sprays need to be applied beginning about second cover (about a month after bloom) and continuing at 10 – 14- day intervals until harvest.

Since sooty blotch and flyspeck are harmless to people and apples, there is no reason to apply fungicides to control them at home – nor do you need to avoid buying apples with sooty blotch and flyspeck, especially for cooking or baking. Most of the fungal colonies can be removed by washing the apples in a mild solution of bleach.

For more information:
Apple Disease Factsheet

Special thanks to Dr. Turner Sutton for helping with this post!

Tuesday, September 20, 2011

Sooty Mold: Ugly... but Mostly Harmless

Have you ever noticed that some of your shrubs have a black mold coating their leaves? Recently, I was at home visiting my parents and my mom was telling me that her crape myrtle was covered in some sort of black mold.  She was very concerned and wanted me to go “check it out.”  I took one look at the bush and knew we were dealing with sooty mold.  
Sooty Mold on Crape Myrtle (Photo By: B. Lookabaugh)
Sooty molds are dark-colored, nonparasitic fungi that grow on insect honeydew.  So what is honeydew - sounds tasty doesn’t it? It is a sweet, sticky liquid produced by aphids, soft scales, mealy bugs, and some species of leaf hoppers.  These insects suck sugary sap from the leaves and excrete honeydew - a mixture of sugars, amino acids, and other organic substances.  Essentially, honeydew is aphid poop!

Soon after a plant is heavily infested with aphids or other sucking insects, leaves are covered in honeydew. The honeydew serves as a nutritional substrate for dark-walled sooty mold fungi. In severe cases, honeydew can drip from infested leaves and stick onto other plants or objects.  When this happens, you might end up with sooty mold on your car, house, propane tank, or patio furniture!
Sooty mold growing on sign beneath infested tree
(Photo By: Whitney Cranshaw, Bugwood)
Like my parents, you may notice more sooty mold problems after it rains. Rain can deposit honeydew even on plants that are not infested with honeydew-producing insects. Since spores of sooty mold fungi are also dispersed in wind and rain, sooty mold problems soon follow.

Sooty molds typically grow in patches on plant surfaces and can be distinguished from other fungi because they are strictly superficial.  The dark mycelium can be wiped clean from the plant surface using a moistened paper towel or your hand. Most sooty mold fungi are harmless to plants. If the mold is extremely abundant, it can prevent leaves from obtaining adequate sunlight and thus reduce plant vigor.  Sooty molds also lessen the aesthetic value of ornamental plants and shrubs. 

Even if sooty mold is extensive, it is best not to apply fungicides.  The mold can be washed away with a forceful jet of water.  You can control the insects that produce the honeydew on which the sooty mold grows with insecticidal soap or horticultural oils.  Horticultural oils also loosen the sooty molds from the plant surface, which speeds up the weathering away process. 
Aphids: Responsible for producing honeydew
(Photo By: B. Lookabaugh
Some plant species and varieties are more prone to aphid problems and thus sooty molds than others. For example, some varieties of crape myrtle are not prone to aphid and sooty mold problems whereas others are very likely to be infested.  See Crape Myrtle Diseases & Insect Pests from Clemson University for more information about sooty molds and a list of recommended varieties.  

For more information:

Friday, September 16, 2011

Rainy Day Weekend Activities

Need something exciting to do this weekend? Join the Plant Disease and Insect Clinic at BugFest and CALS Annual Tailgate.  BugFest is an annual festival that celebrates all things creepy and crawly! This year, the theme is spiders! The festival is held at the North Carolina Museum of Science in downtown Raleigh.  NCSU Department of Plant Pathology (with the Plant Pathology Society of North Carolina and USDA) will once again be featuring the BugBus exhibit.  Grad students will be showcasing mites as plant pests and carriers of plant pathogens.  Kids can come learn about spider mites, fig mosaic virus, rose rosette virus, nematodes, and mummy berry disease.  We will have microscopes set up so kids can view these tiny mites in action. BugFest hours are from 9am to 7pm on Saturday, September 17th.  There are plenty on indoor exhibits, along with lots of spidery themed things to do outside! 

For more information on BugFest, click here.

In addition to BugFest, the Department of Plant Pathology will also be at CALS Tailgate.  CALS Tailgate is an annual event for NC State Alumi.  In addition to a great meal, attendees will get the opportunity to see what our College has to offer from academics, research and extension.  The event is packed around fun events, like a silent auction, live band, departmental displays, children's games, and of course, lots of free stuff.   The Department of Plant Pathology will have a display featuring the Plant Disease and Insect Clinic, Plant Propagation Unit, and Turfgrass Diagnostics.  Head over to our booth to see what our department has been up to and learn about plant diseases. 

After the tailgate, you can head over to watch the NCSU vs. South Alabama game for Military Appreciation Night.  Mark Gottfried, the new men's basketball coach, will be showing his appreciation for our military by parachuting into the stadium!

For more information on CALS Tailgate, click here

Hopefully the cool rainy weather won't ruin your weekend! We hope to see you Saturday!

Tuesday, September 13, 2011

Lions, Tigers, and ORB SPIDERS, OH MY!

With BugFest coming this weekend to Raleigh's Museum of Natural Sciences, we found it appropriate to have a short posting on spiders, specifically orb spiders.  For those of you not familiar with BugFest, it is an annual festival that celebrates all things creepy and crawly! Each year, the festival has a different theme, with this year's theme being SPIDERS.  David Stephan, the Plant Disease and Insect Clinic's resident entomologist, prepared the following post on orb spiders, a common group of spiders found throughout North Carolina.  

Orb Spiders

Spiders are among the most maligned and misunderstood creatures on Earth.  These fascinating arthropods are mostly beneficial to humans as predators of insects and other pests, but the few that are genuinely dangerous to us have given all spiders a bad rap.  Almost all spiders are venomous (except for the species in one obscure family), and almost all are predators.  Very few species are truly aggressive, most opting to freeze or flee in encounters with people.  All spiders produce silk, and many of them use this silk in various ways for prey capture, construction of retreats, courtship, protection of their eggs, etc.

Orb spiders of the family Araneidae are among our largest and most beautiful spiders (oh yes they are!).  They spin those large, radially symmetrical webs that people often find in their yards from mid summer into fall.  Outdoor lights and windows attract many insects, which makes them popular sites for orb spiders and others to spin their webs.  The spider may wait in the center of its web, or in a hidden retreat nearby, ready to rush out and wrap prey that fly into the web.  Most adults will die by winter, after preparing 1 or more sturdy egg cocoons.  Orb spiders are timid and avoid contact with people, but blundering into one of their webs in the dark could give you a heart attack.

Golden Silk Orb Spider, Nephila clavipes
(Photoby David Hillquitst, Bogue Banks, NC)

Our largest NC orb spiders belong to the genera Araneus, Argiope, Neoscona and Nephila.  The mostly tropical species of Nephila include the world’s largest orb spiders, and can build spectacular webs.  The silk that spiders produce has amazing properties of tensile strength and extensibility, and scientists have been studying it for many years.  Researchers in the College of Textiles here at NCSU have worked with orb spiders in the past.  Currently, researchers in our Dept. of Chemical & Biomolecular Engineering are using the latest technology to study, duplicate and even improve on some of those properties of spider silk. 

Golden Silk Orb Spider, Nephila clavipes
(Photoby David Hillquitst, Bogue Banks, NC)

Not enough spider info to satisfy your appetite? Be sure to visit BugFest, Saturday September 17th.  NCSU Departments of Plant Pathology and Entomology have displays with even more info about spiders!

Additional Links:

Special thanks to David Stephan for contributing this post!