Freezing Temperatures Can Be Devastating to Fruit Blossoms

I pasted a chart of killing temperatures for tree fruit.  For more information, see UK publication "Midwest Tree Fruit Spray Guide" ID-92 or click on the chart below.

What You May Not Know about Fire Blight

Fire blight can be a devastating bacterial disease of apple, crabapple, pear, and flowering pear, but disease epidemics are often sporadic.  In fact, optimal conditions must be met for severe disease to occur.  Our current conditions are an indication that fire blight may be severe this spring.

Erwinia amylovora infects trees through flowers.  However, large numbers of bacterial cells must be present during flowering in order for the disease to develop into an epidemic.  The fire blight pathogen favors rain and temperatures above 60˚F.  Under these conditions, bacterial cells multiply quickly.  Thus, if conditions are favorable during flowering, infection can be severe.

Predictive systems are available for growers.  University of Kentucky’s Cougarblight model evaluates the potential for infection by analyzing temperature and leaf wetness data from the previous four days in order to estimate potential risk for infection.  Trees must be in bloom for this predictor to be effective.  Cougarblight is an excellent decision-making tool for growers and can be accessed at http://wwwagwx.ca.uky.edu/plant_disease.html .

Most growers are familiar with shoot blight, the most obvious fire blight symptom in which infected shoots die quickly, causing branch tips to form a distinct crook (photo).  Shoot blight, however, does not result from infection of blossoms.  Direct penetration of bacteria into green shoots or the upper leaves of young shoots after bloom typically causes shoot blight symptoms.

Blighting of shoots, also called shepherd's crook, is the most recognized symptom of fire blight on apple.

Initial fire blight infections occur through flowers.  We call this symptom blossom blight.  Petal browning is the first sign of petal blight, but many growers do not notice it.  Browning of pedicels (stems that attach flowers to stems) follows.  Often, droplets of bacterial ooze can be seen coming from pedicels (photo).  Bacteria quickly travel down the spur and into the twig.  Cankers that form around the spur-attachment site girdle branches, and then branch parts above the canker also die (photo). 

Initial infection by the fire blight pathogen occurs through blossoms.  Notice bacterial ooze come from the pedicel.

Bacteria can spread through flowers and spurs into twigs.  Resulting cankers can girdle entire limbs and branches.

Fire blight control measures include anti-bacterial pesticides applied during bloom.  Applications made after bloom are ineffective.  When fire blight risk is high (warm temperatures combined with rain) during bloom or if fire blight was a problem last year, the following spray schedule should be followed:

1.       Apply fixed copper at silver tip.  Homeowners should not skip this step, as it is their only tool available to combat fire blight.  Do not use copper fungicides after bud break.

2.       Apply streptomycin beginning at pink stage, repeating every 4-5 days, through petal fall.  At least 2 applications are required, but up to 4 sprays may be applied, depending on rain and temperature conditions.  Pay extra attention to susceptible varieties (i.e. Gala, Jonathan, and Rome).  Utilize Cougarblight or MARYBLIGHT predictive systems for assistance.  Mycoshield (oxytetracycline) is also available for management of fire blight but is not as effective as streptomycin.  Neither product is recommended for homeowner use.

Various cultural practices may be implemented to aid in disease management.  Combine these practices with bactericide sprays above for best control.

1.       Select disease resistant or disease tolerant varieties.  Liberty, Pricilla, SirPrize, Enterprise, Gold Rush, and Sundance are recommendations from ID-21.

2.       Prune last year’s cankers and dead wood before bud break.  Burn, bury, or completely remove prunings from the orchard to eliminate the possibility of bacterial cells being carried back to healthy tissue.  Monitor predictive systems.  Know your risk.  Cool temperatures or no rainfall will result in low disease incidence.

3.       Remove flower/fruit spurs immediately after symptoms develop so bacteria cannot continue infection into branches.  Dip tools in 10% bleach, 10% Lysol concentrate, or pure rubbing alcohol after each cut to keep from spreading bacteria.

4.       Do not prune limbs or branches during the growing season.  Trees natural defenses wall off infection sites and stop disease spread.  Remove these branches during the dormant season, instead, when threat of disease spread is lowest.  Removal of all infected wood is critical to prevent spread of inoculum.

More information on fire blight can be found in PPA-34 and PPFS-FR-T-7. 

Spray recommendations for commercial growers ID-92 and homeowners ID-21 are also available online.

Cedar Rusts Threaten Apple

Thanks to this beautiful weather, cedar rust galls are developing.  As galls swell, they produce spores that threaten apple (and sometimes crabapple and hawthorn).

 Images of swelling galls were taken from cedar in Lexington on Monday March 19.  Warmer temperatures in western KY allowed even earlier gall development.

These galls indicate that rust pathogens are releasing, or are preparing to release, infective spores.  Growers should protect trees with fungicides that are proven to be effective against rusts (listed below).  Once diseases symptoms develop on apple, it is often too late for control. 

Here in Kentucky, 3 types of cedar rusts affect apple: 

1.       Cedar-apple rust produces large brown galls on cedar and other species of Juniperus.  Soon after a rain, galls produce slimy yellow or orange “horns” that are made up of infective spores (basidiospores).  These spores immediately infect apple.  Upon infection, the disease causes yellowish leaf spots with red rings (called halos).  Leaf yellowing and leaf drop follow.  Infected fruit develop large spots near the calyx end.  These fruit are often stunted and may drop prematurely. 

Gall beginning to form "horns."

"Horns" contain infective spores in a gelatinous material. 
This is an old photo, but we can expect to see this fully-developed stage soon in central KY.
Never wait until this stage to start a spray program on apple.

2.       Cedar-quince rust produces orange swellings on twigs.  Spores produced in these slimy lesions affect apple fruit, but not leaves.  Infection of blooms and young fruit occurs early in the season, but symptoms do not develop until fruit mature.  Diseased fruit are puckered and have spongy lesions at the calyx end.

Sometimes growers must look closely to recognize rusts, so scouting is important.

3.       Cedar-hawthorn rust forms galls similar to those of cedar-apple rust, only smaller.  Spores produced from short “horns” infect apple, crabapple, and hawthorn.  Leaf spots on apple appear similar to those caused by apple cedar rust, and can cause defoliation.  Fruit infection is not common.

Often, many rust types occur on the same tree.

Fungicides should be used as protectants to prevent rust pathogens from infecting.  After symptoms develop on apple, it is often too late for control. 

Commercial and homeowner fungicide recommendations are listed below.

Commercial Fungicides for  Management of Rust on Apple
Bayleton	Triadimefon
Dithane	Mancozeb
Indar	Fenbuconazole
Inspire Super	Difenoconazole
Pristine	Boscalid + pyraclostrobin
Procure	triflumizole
Rally	Myclobutanil
Sovran	Kresoxim-methyl
Topsin-M	Thiophanate-methyl
Homeowner Fungicides for  Management of Rust on Apple
Dithane	Mancozeb
Immunox Multi-purpose	Myclobutanil
Mancozeb	Mancozeb

More details for management of cedar rusts can be found in Kentucky Pest News or PPA-23 (http://www.ca.uky.edu/agc/pubs/ppa/ppa23/ppa23.pdf ).

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.

3 things you should know about thousand cankers disease

1.       TCD is fatal to walnut trees.  Thousand cankers disease (TCD) is a fungal-insect complex that infects walnut and butternut.  Symptoms begin with yellowing of leaves during summer.  Gradual decline follows as limbs die.  Within 3 years from symptom development, entire trees die.  There is no treatment for thousand cankers disease (TCD) - no preventative and no cure.

2.       TCD is at our backdoor.  The disease has already been detected in Tennessee, Virginia, and Pennsylvania.  TCD will be devastating if/when it enters our state.  Every Kentuckian can help stop the spread by monitoring local walnut trees.  Look for leaf yellowing or branch “flagging,” as well as overall tree decline.  Thinning canopies or dieback from the tops down is often paired with water sprouts extending from bases of trees.  Small, dime to quarter-size cankers form under bark but may not be detected without peeling back bark.

3.       TCD moves with wood.  Small beetles carry the fungus from tree to tree, but trees are moved by humans.  Reduce the likelihood of transmission of TCD by using locally processed lumber and firewood.  Never move wood from quarantined areas.  Contact authorities before removing dead or unhealthy walnut trees.

Submit samples of suspected diseased trees to your UK Extension county agent by following sampling instructions from PPFS-OR-W-15.  Additional information on TCD can be found at:        NA-PR-02-10 from the National Forest Service, National Pest Alert, or the photo gallery at Forestry Images.  If you suspect TCD, contact your local UK Extension agent.