The Heartbreak of Botrytis

Decay at the terminal portion of the flowering stem of
poinsettia, due to infection by Botrytis cinerea

Dark blotches are bract
infections by B. cinerea

On Monday, December 16th, we received a sample of poinsettias that for all commercial purposes had been ruined by Botrytis blight. Botrytis cinerea is a pernicious and ubiquitous fungus that particularly infects wounded or senescent tissue such as old flowers. From this foothold it can spread to other plant parts. When the fungus sporulates, the colors of the black conidiophores (spore-bearing threads) and white conidia (spores) combine to give the appearance of a gray mold. The spores are easily carried around on air currents.

Botrytis cinerea can wreak havoc with many different host species, even causing canker on rose canes and fruit rot on plants such as strawberry. More specialized species of Botyrtis also exist. One is Botrytis elliptica, which affects primarily lilies. Botrytis tulipae causes a disease called "fire" on - you guessed it - tulip.

Closeup of conidiophores and conidia of Botrytis cinerea,
growing on cyathia (true flowers) of poinsettia. Black bar = 1mm.

The Achilles heel of Botrytis is its need for abundant moisture. In greenhouses, care must be taken to ventilate, even for a while after sunset, in order to keep the relative humidity down. Watching watering practices (timing, drainage) is also important. Fungicides are sometimes needed. Another essential element in Botrytis management is prompt removal of dead plant material from the house. For more information on Botrytis in greenhouses, see our blog from May 18, 2012.

Pansy bed at NCSU in early 2012. Botrytis blight.

Be on the look out for Botrytis blight on pansies in the landscape, especially in the late winter and early spring. To prevent problems, avoid overhead watering if possible, and make sure the plant spacing and surrounding shrubbery allow for good airflow. For more information on Botrytis in the landscape, see the May 31, 2013 blog.

Light colored dead blotches on flowers, spreading to leaves,
are a hint that you might have Botrytis blight in your pansies.

Just a reminder to check our holiday closing schedule. We look forward to seeing you in 2014!

Mike Munster and Kelly Ivors

Sample of the Week: Poinsettia scab

Rapid elongation of poinsettia stem infected
with Sphaceloma poinsettiae

Poinsettia scab, caused by the fungus Sphaceloma poinsettiae, was found on a sample from a commercial greenhouse this week.  It has been six years since the PDIC last diagnosed this disease in a North Carolina poinsettia crop. As the name implies, this fungus causes leaf spots and stem lesions, but the most noticeable effect is an abnormal elongation of the poinsettia stem. The purple leaf spots may develop a light tan center, and they sometimes have a yellow halo. The surface of the spot is characteristically puckered, which is best seen under magnification. An olive-colored, velvety layer of spores may be present on the spots and stem lesions. These spores are spread to other plants via water splash. Long-distance transport occurs on infected planting material. This disease cannot survive between seasons in North Carolina in the absence of a poinsettia crop. For a good summary of the disease, see the 2001 APSnet publication by Mike Benson et al. Growers should be sure they get clean stock and should scout points for leaf and stem symptoms. Keeping leaf wetness to a minimum will help reduce the advance of the disease. Apply azoxystrobin (Heritage), trifloxistrobin (Compass O), triflumizole (Terraguard), or triadimefon (Strike) to protect plants. An interesting side note is that while this fungus is a problem for poinsettia producers, it has been studied as a possible biocontrol agent for wild, weedy poinsettia relatives in the tropics. 


Special thanks to Dr. Kelly Ivors for contributing to this post.

Close-up of poinsettia stem showing scab lesions

Scab lesions on poinsettia leaf, caused by Sphaceloma poinsettiae

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

Problem Pythium?

Have you been having a problem with Pythium root rot at your facility? Do you think that you may have resistant isolates? If so, you can contact the Plant Disease and Insect Clinic (link below) for more information about sample submission procedures.  I am going to be actively sampling North Carolina greenhouses for Pythium isolates.  If you would like us to sample your greenhouse or test your plants for mefenoxam resistance we might be able to add your facility to our list.  Please contact the clinic for more information. 

http://www.cals.ncsu.edu/plantpath/extension/clinic/

  

A Little Bit About Pythium

About Pythium

Pythium species are  “water molds” that produce swimming spores called zoospores. There are many species of Pythium.   Some species are saprophytes or weak pathogens that mostly decay dead root tissue.  Often, Pythium nibbles on feeder roots of plants. Under the right environmental conditions, however, some Pythium species become destructive pathogens that rot and kill the roots of plants, resulting in stunted growth or death.  The pathogen is favored by wet conditions, such as when media does not drain properly or when weather events prevent the soil from drying out completely.  Pythium can also be more problematic when the host plants are stressed.

             Pythium can be a problem on many annual and perennial hosts and is extremely destructive to greenhouse production of poinsettias and geraniums. Pythium can be introduced into greenhouses on infected plugs or infected plant material.  It can also be a year-round “resident” hiding on dirty plant containers, equipment, benches, or even in irrigation water, waiting for the conditions that favor the pathogen and disease development. 

Leaf curling associated with Pythium root rot on poinsettia

Stunted growth symptom of Pythium root rot on poinsettia

Managing Pythium Problems

Sanitation is very important in managing root rot because Pythium produces survival structures, called chlamydospores, that are able to survive for long periods of time on infected plant material or dirty benches and pots.  There are some fungicides that are effective against Pythium and other water molds (oomycetes).  Some of the most popular oomycete fungicides contain the active ingredient mefenoxam.  In recent years, greenhouse growers have seen an increase in oomycete resistance to mefenoxam, resulting in poor disease control.  Some good practices that limit the occurrence of fungicide resistance include:

• Time spray applications when the pathogen is weakest or most vulnerable to application
• Use a fungicide registered for your pathogen that is proven to be effective in controlled settings
• Alternate active ingredients in your spray programs
• Use only the labeled rates listed on the container and do not over-apply the fungicide
• Incorporate new fungicides into your spray program as they become available

Unfortunately, even when using good application practices, sometimes the pathogens still develop resistance. 

Research Goals

The purpose of my research is to determine which Pythium species are present in North Carolina greenhouses and to evaluate their abilities to be effective pathogens in the greenhouse environment.  This will involve morphological and molecular species characterization, pathogenicity assays, growth chamber studies, and checking for mefenoxam sensitivity.  

At the plant disease clinic, we routinely run root assays to determine the presence of Pythium.  Currently, we are not able to determine which species of Pythium is present and whether it is pathogenic or just saprophytic.  My research will allow the diagnosticians at the disease clinic the opportunity to expand their Pythium diagnosis to be more beneficial to the grower.  We will be able to tell the grower whether the Pythium we isolated from their plants is resistant to mefenoxam and if so, offer alternative control options.  

Fungicides are expensive.  Routine mefenoxam screening will help growers develop effective fungicide programs so they are not spraying money down the drain.