Songs of the Season, with a Twist

Are you tired of nonstop carols yet? While not as popular as Santa, plants star in many songs of the season – and some of them have special associations for us here at the PDIC. Number one on the list must be “The Christmas Song,” aka Chestnuts Roasting on an Open Fire. This song reminds us of the blight that devastated the Eastern forests in the last century, leaving us with no chestnuts to roast. “Oh, Christmas Tree” (Oh, Tannenbaum) makes us think of North Carolina’s beautiful Fraser firs and the root rot they must escape before they can end up in our living rooms. Mistletoe figures prominently in “I’ll be Home for Christmas” and holly pops up in “Have a Holly Jolly Christmas” and “The Holly and the Ivy.”

But here’s a song that puts me in the holiday spirit even though the subject and the tune have nothing to do with snow, presents, Santa and all the rest. That’s because for many years, NCSU Plant Pathologists sang it lustily at every gathering, including the annual holiday party. I promise you have never heard it on the radio. Enjoy!

Photo by David Langston, Univ. Georgia

For a nice write up about managing root-knot nematodes, check out this article by Pender County Agent Charlotte Glen.

THE NEMA SONG

(To be sung with gusto to the tune of “The Battle Hymn of the Republic”)

A nematode was chewing on a young tobacco root.

Her keen and piercing stylet was protruding from her snoot.

Her salivary glands were squirting out the poison juice.

The galls keep hanging on.

(Chorus)

We must fight the nasty nema,

We must fight the nasty nema,

We must fight the nasty nema,

Rotate crops and pray.

A cotton plant must suffer on a hot and sunny day.

It transpires too much water when its roots are chewed away.

If we don’t lick the nema we will really have to pay.

The galls keep hanging on.

(Chorus)

Meloidogyne incognita,

Meloidogyne incognita,

Meloidogyne incognita,

Fumigate and pray.

The cortex cells are swelling to a giant size to stay.

The female’s sac’s protruding in a morbid sort of way.

The blind roots now are forming and the root knot’s on the way.

The galls keep hanging on.

(Chorus)

We must fight the nasty nema,

We must fight the nasty nema,

We must fight the nasty nema,

Rotate crops and pray.

Lyrics by Dr. G. B. Lucas

Poinsettia: Some Common Diseases of the Christmas Flower

Fig. 1. Poinsettia- the Christmas flower
(with permission Benson, et al. 2002. Plant Health Progress
doi:10.1094/PHP-2002-0212-01-RV).

Poinsettia, the Christmas flower, (Fig. 1) was introduced to the United States from Mexico in 1825 by the first U.S. Ambassador to that country, Joel Roberts Poinsett of Greenville, South Carolina (Fig. 2). 

Fig. 2. Joel Roberts Poinsett, first US Ambassador to Mexico
(with permission Benson, et al. 2002. Plant Health
Progress doi:10.1094/PHP-2002-0212-01-RV).

Commercial interest in poinsettia as a potted plant grown in the greenhouse did not get much attention until the 1950s and 60s, when breeding programs developed plants with stiffer stems, multiple shoots at each pinch point, larger flower bracts, and better keeping qualities. Today, poinsettias come in a variety of forms and bract colors. The value of poinsettias is about $145 million per year in the United States with about $17 million coming from North Carolina growers (Fig. 3). Although poinsettias are the Christmas flower, it is only consumer preference that limits year round sale.

Fig. 3. Poinsettias in commercial production
as the flower bracts are beginning to turn red.

Poinsettias are propagated vegetatively by cuttings taken from stock plants usually beginning in late June and early July just when greenhouse temperatures are highest. Cuttings are propagated typically in either polyfoam wedges, rockwool, or direct stuck in the finish size pot. Regardless of propagation strategy, cuttings must be misted several times a day to keep them from wilting until roots form on the stem of the cutting (Fig. 4). 

Fig. 4. Propagation of poinsettia cuttings in polyfoam rooting
wedges under an intermittent mist system.
Note droplets of water on foliage from misters.
(Photo E. Lookabaugh) 

During propagation, growers must avoid or prevent a number of plant diseases that can attack the cuttings. Under extreme moisture conditions, the soft rot bacterium, Erwinia carotovora attacks the cut end of the stem resulting in a mushy, watery rot that kills the cutting (Fig. 5). 

Fig. 5.  Erwinia soft rot has collapsed these poinsettia
cuttings in propagation
(with permission Benson, et al. 2002. Plant Health
Progress doi:10.1094/PHP-2002-0212-01-RV).

Even when misting systems are functioning normally, Rhizoctonia stem rot caused by R. solani can cause a canker on the lower stem that kills the cutting (Fig. 6, 7). When cuttings are stuck directly in potting mix in the finish pot, Pythium rot caused by several species of Pythium as well as Rhizoctonia stem rot can develop, if these pathogens are introduced by faulty sanitation procedures. Healthy cuttings root in about 4 to 6 weeks depending on temperature, if plant diseases do not develop.

Fig.  6. Poinsettia cutting in a polyfoam propagation
 strip with Rhizoctonia stem rot.  Note brown stem
 lesion at bottom of cutting near foam surface. (Photo Mike Benson)

Fig. 7. Rhizoctonia stem rot. Two close ups of a stem lesion with
the white mycelium of the Rhizoctonia pathogen present
(with permission Benson, et al. 2002. Plant Health
Progress doi:10.1094/PHP-2002-0212-01-RV).

Cuttings once rooted in polyfoam wedges or rockwool must be transplanted to a soilless potting mix in a pot to finish for retail. The most important foliar disease growers must guard against in this stage of production is gray mold caused by Botrytis cinerea.(Fig 8). As the plant canopy grows and fills in, high humidity in the microclimate of the canopy is an ideal environment for gray mold. Growers must ventilate greenhouses properly to avoid high humidity and some even use bottom heat via air tubes under the greenhouse bench to help dry out the plant canopy. Fungicide sprays may also be used to prevent gray mold.

Fig. 8. Botrytis blight on foliage. Note dead tissue and abundant
 ‘gray mold’ sporulation on the infected tissues. This infection developed
 inside the plant canopy where humidity was high favoring
pathogen infection and sporulation. (photo Mike Benson)

In the 1990s powdery mildew caused by Oidium spp. caused severe losses for many growers. The disease was particularly devastating because it often times did not develop until the plants already had color in the flower bract and by that time the grower had most of the expense of growing the crop already invested in it (Fig. 9). Growers also were reluctant to use fungicides sprays for powdery mildew control in the late stages of production because of spray residue concerns on the flower bracts. The disease has not been a problem in the last decade, however, due to changing cultivars and better management practices.

Fig. 9. Colonies of powdery mildew on leaves (left) and flower bracts (right)
(with permission Benson, et al. 2002. Plant Health Progress doi:10.1094/PHP-2002-0212-01-RV).

Scab is a stem and foliage disease caused by the fungus Sphaceloma poinsettiae that can occur periodically, resulting in unsalable plants. The most striking symptom of scab is the extra long stems produced by plants infected with this fungus (Fig. 10). Leaf spots also develop on infected plants. Outbreaks of scab usually occur when the pathogen is introduced with poinsettia stock material arriving from Central and South America where the fungus occurs throughout the year.

Fig. 10. Abnormally elongated stems of poinsettia due to scab disease
(with permission Benson, et al. 2002. Plant Health Progress doi:10.1094/PHP-2002-0212-01-RV).

The most important root disease affecting poinsettia is Pythium root rot caused by several species of Pythium the most common being P. aphanidermatum, but P. irregulare, P. cryptoirregulare and P. ultimum also cause loss. The fungus-like Pythium survives between crops in infected plant material from previous crops whether they are poinsettia or not. Without thorough sanitation between crops Pythium can be re-introduced to the new poinsettia crop by infested crop debris or through the irrigation system. The most common symptom of Pythium root rot is stunting of the plant as it fails to keep pace with the growth of healthy plants (Fig. 11). 

Fig. 11. Stunting of poinsettia plants caused by Pythium root rot during finishing.
 Note healthy plant in foreground compared to stunted, disease plants scattered throughout.
(Photo Mike Benson)

Under severe disease pressure, the foliage of plants with Pythium root rot develops wilt symptoms and does not recover with irrigation. Affected roots are discolored (Fig. 12). This disease can attack the crop at any time from propagation through finishing. Pythium root rot occurs in greenhouses regardless of location as some Pythium species are aggressive at low temperatures and others at high temperatures. Overwatering favors Pythium root rot. Fungicide drenches are commonly used to prevent the disease.

Fig.  12. Pythium root rot of poinsettia on a newly-transplanted rooted cutting. 
Wilt symptoms (left) and close up of discolored roots with root rot from same plant (right).
(Photo Mike Benson)

Phytophthora root rot caused by P. drechsleri and P. nicotianae also can attack poinsettia during the finishing stage resulting in unsalable plants. Phytophthora is fungal-like pathogen similar to Pythium. Symptoms are the same, as well. Unlike Pythium, however, these Phytophthora pathogens can also splash onto poinsettia foliage causing a blight disease. Like Pythium root rot, overwatering favors this disease too and fungicides are commonly used to prevent the disease.


For a detailed history of the poinsettia and poinsettia diseases click here

Post prepared by Mike Benson

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!