Fire Blight Damage on Flowering Pear

Homeowners continue to question me concerning management of fire blight in flowering pear, crabapple, cotoneaster, hawthorn, and pyracantha.  Shepherd’s crooks and spur dieback occurred in March or April, as pathogenic bacteria infected flowers or young shoots.  Often, homeowners do not notice damage until later in the season, and inquiries continue for weeks after damage occurred.

There is no management option during this time of year, as the fire blight bacterium is not currently active.  Hot summer temperatures suppress bacterial growth, and plants are able to compartmentalize and wall off spread.  Thus, visible symptoms are the result of early infections. 

Current recommendations indicate that pruning of blighted twigs and cankered branches should be delayed until winter when risk of disease spread is lowest.  Under certain circumstances, homeowners or commercial landscape contractors may choose to prune infected branches during the growing season.  Only young, vigorous trees should be considered, and care should be taken to prevent bacterial spread.  Always avoid working with wet plants.  Cut branches at least 6 to 8 inches below cankers, disinfesting pruners between each cut (10% Lysol disinfectant, 10% bleach, or rubbing alcohol).  Ideally, winter pruning is recommended. 

Disease management includes both cultural practices and preventative bactericides.  Because the fire blight bacterium overwinters in cankered branches, removal of diseased plant tissue before bud break (mid to late winter) is critical.  Copper applied as buds swell (late dormancy or silver tip) reduces build-up of bacterial cells, especially during warm rainy spring seasons.  Streptomycin applications are only recommended for fruiting apple and are not labeled for use in the landscape. 

See earlier posts for more information on the biology of the fire blight bacterium and for management in orchards.

Top Tips for Combatting Crown Gall on Grape

In extreme cases, crown gall can overwhelm vines or trunks. 

Severely diseased vines such as this one should be removed immediately.

Crown gall disease of grape is caused by the bacterium Agrobactgerium vitis (A. tumefaciens biovar 3).  Infection and colonization of grape vines by this bacterium are best prevented by cultural practices, which are critical for prevention of crown gall disease. 

Crown gall symptoms are characterized by distinct tumor-like overgrowths on roots, trunks, and vines.  New galls that form during early summer are light-colored, but they turn dry and corky by late summer.  Expansion of galls results in girdling of vines and roots.  Suppression of water and nutrient uptake by galls can lead to either rapid vine death or gradual dieback and susceptibility of adverse environmental conditions.

Crown gall bacteria are soilborne, but they survive indefinitely in diseased plants.  Infected tissue serves as reservoirs for bacterial cells that can infect other plant parts or healthy plants. 

·          Practice sanitation, remove all cuttings from vineyards immediately after pruning.

o   Remove diseased vines.  Remove as much of the rootstock as possible, as bacteria survive for long periods in root fragments.

o   Sometimes cutting out diseased wood is effective, but bacteria are systemic (they colonize entire plant vascular systems).  Removing all sources of inoculum is highly recommended, but removing entire vines is not always possible.  If vines are trained with two trunks, a diseased section can be removed while the other section remains productive.

The crown gall bacterium enters through wounds, such as bark cracks caused by freeze damage.  In fact, Vitis vinifera, or French varieties, are most susceptible to cold injury and are therefore most susceptible to crown gall disease.   

·          Protect graft unions from winter injury.  Freeze wounds are the primary sites of entry, and V. vinefera are extremely susceptible to freeze injury.

·          Insect wounds are also ideal openings, so control of phylloxera and other insects can reduce numbers of these entry points.

·          Beware mechanical damage (string trimmers and other equipment).  Again, the crown gall bacterium requires wound sites to enter plant tissue.

                               

Once established, bacterial cells ooze from galls on plants.  These cells can be spread by water splash/runoff, pruners, hands, and mowers. Simply touching an oozing wound provides inoculum for the next surface.

·          Sanitize pruners after each cut when pruning infected plants.  A 10% bleach or 10% Lysol solution in a small bucket makes a great dip.

·          Limit overhead irrigation to reduce splash and runoff of any kind.

·          Start with clean stock.  Use reputable suppliers.  This step is critical, especially with V. vinifera.

o   Bacterium is systemic and can be transferred from mother plants into cuttings.

o   Use certified bacterium-free stock.  It is worth the money in the long run.

Use disease resistant cultivars, if possible.

o   V. vinifera are highly susceptible, while table grapes typically show the lowest disease incidence. 

o   Plant highly susceptible cultivars far away from other plants.  Segregating plants is worth the effort in terms of isolating diseased plants and treating diseased ones.

Biological control agents are available for treatment of crown gall.  However, they are not a cure that growers should depend upon.

o   Agrobacterium radiobacter strain K-84 is effective in reducing galls in vines infected with some species of Agrobacterium, such as A. tumefaciens, but it is not effective against A. vitis. Strain 84 is available as Galltrol A or Norbac 84C and may be used as a pre-plant dip. http://agbiochem.com/Galltrol.aspx

o   Products such as Gallex can be applied to existing galls on infected vines (with a paintbrush) in summer or fall to reduce gall formation. http://agbiochem.com/GallEx.aspx.   Multiple applications will control, but not cure, crown gall disease.

o   Other strains of Agrobacterium, such as A. vitis strain F2/5 will be released soon.  It is being examined as a protectant and looks promising.

o   Copper fungicides can be good protectants, especially if used often.  However, some grape cultivars are sensitive to copper.  See Table 14 in the Midwest Grape Production Guide for a listing.  http://ohioline.osu.edu/b919/0010.html

Crown gall disease can be easy to overlook.  Sometimes it is necessary to peel back bark in order to see small galls.  At this stage, treatment of established, productive vines may be warranted.  Young infected vines, on the other hand, are often replaced.

Resistant Fire Blight in Apple

Strep-Resistant Fire Blight Found In New York

A strain of streptomycin-resistant fire blight has been detected in New York apple orchards.

January 10, 2012  from http://www.growingproduce.com/article/24655/strep-resistant-fire-blight-found-in-new-york

 

Cornell plant pathologists have issued a warning to New York apple and pear growers after discovering a strain of fire blight that is resistant to such traditional treatments as the antibiotic streptomycin.
For 50 years, the disease has been kept at bay using the antibiotic, but streptomycin-resistant strains of the disease were recently found in four locations in Wayne and Ontario counties.
“This is a serious situation that we need to manage effectively. The rapid identification of this outbreak positions us to implement a coordinated plan leading up to next year’s growing season,” said Herb Aldwinckle, professor of plant pathology and plant-microbe biology at Cornell’s New York State Agricultural Experiment Station (NYSAES) in Geneva.
In coordination with colleagues at NYSAES and Cornell Cooperative Extension, Aldwinckle is developing guidelines growers can follow to reduce the threat. Upcoming sessions at the Fruit and Vegetable Expo and CCE Fruit Schools in several counties will also provide growers with opportunities to learn more about the outbreak and possible solutions.
Fire blight is caused by the bacterium Erwinia amylovora, earning its name from the scorched appearance of infected leaves and branches. Its symptoms include blackening of flowers and young leaves, resulting in crop loss and even death of trees.
This current infection was first spotted in Wolcott, NY, where a persistent suspected fire blight infection evaded a streptomycin spray regime. Samples were immediately brought to Aldwinckle’s lab.
Sampling was necessarily limited in late fall, so the full extent of the outbreak will be unclear until additional samples are studied, but the latest results indicate there are several outbreaks of strep-resistant fire blight in Wayne and Ontario counties.
Aldwinckle said the fact that one of the sites is a nursery does complicate the management plan. Fortunately, that nursery has a strict rogueing program — in which infected plants are removed from the soil and destroyed — that should minimize the chance of spread, he added.
However, Aldwinckle noted that some infections might not be immediately visible, so there’s a possibility that a small number of trees with the strain were inadvertently shipped to growers.
He advises all growers to examine their trees carefully for any symptoms of fire blight that may develop after planting. These recommendations apply to all trees purchased, with particular attention for those from New York and Michigan nurseries, where strep-resistant fire blight is known to occur.
He is also exploring other ways to eradicate the blight.“The antibiotic kasugamycin is as effective as streptomycin, and in some ways it is a more appropriate antibiotic because it is not used in human or veterinary medicine,” said Aldwinckle. “Kerik Cox, an assistant professor of plant pathology at Cornell, is leading an application for a Section 18 emergency registration from the EPA for growers to use Kasumin [commercial kasugamycin] next spring.”

Source: Cornell University College of Agriculture and Life Sciences