Tuesday, November 29, 2011

Phytophthora Root Rot on Fraser Fir

Fraser Fir: The "Cadillac" of Christmas Trees
Fraser fir is a highly desirable Christmas tree species grown in the mountains of Western North Carolina and elsewhere across the United States. As a leading producer of Christmas trees, North Carolina growers must overcome weeds, insects, and disease to produce that beautiful tree you buy on the Christmas tree lot. 
Fraser fir nursery bed where seedlings are grown up to 
5 years before transplanting to the field.  (Photo: Mike Benson)

Typical Fraser fir planting in Western North Carolina 
after about 5 to 6 years in the field. (Photo: Mike Benson)
Although growers have good management tools for weeds and insect pests, Phytophthora (pronounced fy-TOF-thor-uh) root rot threatens long-term production of Fraser fir. The pathogen, Phytophthora, produces zoospores that can swim to tree roots in saturated soil or run-off. Throughout the 11 to 15 years or more that is takes to produce a 6 to 8 foot tall Fraser fir, Phytophthora root rot is always a threat. 
Fraser fir planting that experienced Phytophthora root rot that started on trees at top of 
ridge. Over several years, trees downhill from ridge became infected and died.  
The Phytophthora pathogen produces spores that are carried in water run-off, so
the ‘wedge’ shaped area of missing trees developed in the downslope drainage 
area of the field as trees were killed by the disease.  (Photo: Mike Benson)
Fraser fir planting  after about 6 years in the field. Note patch of trees killed by 
Phytophthora root rot in far edge of field. (Photo: Mike Benson)
Lower branch ‘flagging’ may be the first indication that a Fraser fir has Phytophthora root rot.  Once the Phytophthora pathogen has infected enough of the root system that the plant can no longer transport water and nutrients  adequately, the foliage becomes yellow then turns reddish brown a short time later.  The tree will eventually die from the disease. 
Lower Branch "Flagging" (Photo: Mike Benson)
Root systems from a healthy Fraser fir transplant (right) compared to an infected 
and dying transplant (left).  Roots killed by the Phytophthora root rot pathogen 
are dark, reddish brown in comparison to root tips on healthy plants that are white. 
(Photo: Mike Benson)
Growers manage the disease by planting healthy fir transplants into fields that are well drained. Once disease develops in a tree, however, that tree is lost. Future production in that area of the field is also threatened because the pathogen survives many years in soil.

In the future, forest tree breeders and plant pathologists hope to develop a Phytophthora resistant fir with qualities equal to the Fraser fir. So when you buy that fresh cut Christmas tree this year, just remember that the Christmas tree grower was able to overcome a lot of potential problems to provide that tree for you.

For more information, click here

Special thanks to Mike Benson for writing this post!

Friday, November 18, 2011

Sample of the Week: Papaya Ringspot Virus on Watermelon

Recently, we received a very interesting watermelon sample that was infected with Papaya Ringspot Virus (PRSV).  This is only the second time we have seen this disease in the clinic.  PRSV causes diseases on cucurbits and papaya plants worldwide.  The most obvious symptom of PRSV infection is the formation of sunken ring spots on the fruit.  These spots are very diagnostic of the disease.  Other symptoms include mosaic or mottling of the leaves and oily streaks on the stems and petioles. In some cases, the leaves may be severely narrowed, which gives them a “shoestring” like appearance. 
Rinspot symptoms of PRSV (Photo by: Jennifer Pries)
PRSV is a member of the potyvirus group of viruses and is transmitted by aphids. The virus is vectored in a nonpersistant manner, meaning it does not replicate in the aphid vector.   Aphids feed on infected plants, and within seconds or minutes they hop to healthy plants and transmit the virus as they feed.  The virus can spread very quickly through fields. 

The two major strains of PRSV are PRSV-W and PRSV-P.  PRSV-W (the watermelon-infecting type) used to be known as Watermelon Mosaic Virus I, and can only infect cucurbits.  PRSV-W is the major strain found in the Southeastern US, but it is also present in tropical areas where PRSV-P is present.  PRSV-P (the papaya-infecting type) is only found in tropical and subtropical areas were papayas are grown, but PRSV-P can also infect cucurbits.  Controlling this disease is somewhat difficult, especially in papaya.  Genetic resistance to PRSV is available in some commercial varieties of cucurbits and is the most effective way of controlling the disease.  Unfortunately, resistance has not been found in watermelon.  Aphid  control is not practical because of the nonpersistant manner in which the virus is transmitted.  

For more information,
For resistant varieties,

Friday, November 11, 2011

Sample of the Week Sweetpotato Scurf

With Thanksgiving on the way, we decided to highlight a common disease of sweetpotatoes, but don’t worry, it wont affect the taste of your sweetpotato casserole! Scurf is caused by the fungus Monilochaetes infuscans.  The fungus only grows in the sweetpotato skin and produces spores on the surface of the skin.  As shown in the photos, the dark splotches on the skin are areas infected with the fungus.  The fungus does not penetrate below the skin, so the sweetpotatoes are still edible, but because of the dark discoloration that results, diseased sweetpotatoes have lower market values. 

Healthy sweetpotatoes are infected by spores present in the soil.  The disease can also spread through contaminated crates, baskets, and storage houses.   The fungus does not attack the above-ground portion of sweetpotato plants, and does not attack other hosts so rotation is an effective way to prevent scurf problems.  Scurf is usually worse during rainy seasons.  

For more information and control recommendations, click here

Friday, November 4, 2011

Sample of the Week: Cucurbit Downy Mildew on Greenhouse Cucumbers

This week’s sample is cucurbit downy mildew on greenhouse cucumber.  Cucurbit downy mildew affects all members of the cucurbit family: watermelons, cantaloupes, squash, pumpkins, and cucumbers.  Disease is favored by long periods of high humidity and mild temperatures, which unfortunately, describes most nights in the Southeast during the production season and in greenhouses year round.  Downy mildew occurs every year in North Carolina, and in recent years has become increasingly destructive on cucumber.  
Greenhouse Symptoms (Photo: PDIC Database)
The most obvious symptoms of downy mildew are small angular spots on the foliage, with older leaves generally being infected first. “Angular leaf spots” describe a type of symptom where the spots are defined by the boundaries of leaf veins, so that the shape of the spots is angular rather than round or blotchy. Downy mildew spots appear pale green to yellow as first, eventually becoming brown and necrotic with age.  
Angular Leaf Spots (Photo: PDIC Database)
During humid conditions, brown to purplish fuzzy growth can be observed on the underside of the foliage.  This downy growth is actually sporulation of the pathogen. 
Fuzzy Sporulation (Photo: Shawn Butler)
Dichotomously branched sporangiophores with lemon-shaped sporangia are visible when view through a microscope.  As infection progresses, leaves will eventually turn brown and curl upwards.  The leaves are the only part of the plant affected.  Downy mildew infections result in yield loss and misshapen fruits.  Damage from foliar infections also increases sunlight exposure on fruit and leads to sunscald.  

Cucurbit downy mildew is caused by Pseudoperonospora cubensisP. cubensis belongs to a group of fungus-like organisms called oomycetes (or water molds).  This group is also home to other aggressive plant pathogens including Phytophthora and PythiumP. cubensis is an obligate parasite, meaning it requires living host tissue to survive and reproduce.  North Carolina winter temperatures are too cold for the pathogen to overwinter, so it dies out every winter. Unfortunately, Southern Florida has the perfect combination of mild winters and wild cucurbit plants. The pathogen survives all year in Florida and inoculum builds up on wild plants.  Spores are windblown and can travel long distances on air currents. Summer weather events, like hurricanes, act as the perfect mode of transportation.  Like birds, downy mildew spores fly north for the summer and end up on our cucurbits! In this particular situation, the greenhouse plants probably became infected as spores from nearby field cucumbers blew into the greenhouse over the summer.

For more information on this disease, click here

For more information on controlling this disease, click here

For more information on forecasting this disease, click here