Alert - 6: June 6, 2006

Fire blight

We have seen a couple of fire blight samples in the diagnostic lab recently.  The disease is most active during wet weather when temperatures are about 65-85 F.

Fire blight is caused by a bacterial species called Erwinia amylovora.  It affects many plants in the rose family including apple, crabapple, pear (including ornamentals), hawthorn, quince, and cotoneaster.  The pathogen survives the winter in cankers.  In the spring the bacteria ooze out of cankers and are spread by rainsplash, wind, and insects to blossoms.  This causes “blossom blight.”  The pathogen can also infect succulent new leaves and shoots, especially tissue that has been wounded by pruning or hail.  The diseased shoots become brown (on apple) or black (pear) and often have a curled top, referred to as a shepherd’s crook (see photo).  Infected shoots can also produce sticky, orange ooze (see photos).

The bacterium can move systemically within the infected tree.  In commercial apple plantings the bacterium is most damaging when it moves down into the rootstock, eventually killing the entire tree.

There are several cultural practices to help manage fire blight.  The pathogen is most infective on lush, new growth.  Avoid fertilization, especially later in the season.  When the shoots harden off and stop growing they are much less susceptible to infection.

Diseased areas can be pruned out in the summer, but do not prune in wet weather.  Make the cuts at least 12-18 inches below the lowest visibly diseased tissue.  Disinfect tools between each cut with 70% ethanol or 10% bleach (if you use bleach be sure to clean and oil your tools after use to avoid damage). 

Pruning in winter is often a simpler, easier method.  Remove dead shoots and cankers.  As with summer pruning make the cut below visible diseased tissue, at least 8-10 inches to be safe, and sanitize tools between cuts

Chemical controls are available and are most effective at bloom so this year we have passed the window of chemical control.  If you are concerned about fire blight next spring you might consider applications of the antibiotic streptomycin during bloom.  Streptomycin needs to be applied about every 4 days throughout the entire bloom period.  There is at least one streptomycin product available for home apple growers that is sold by the name Fertilome. 

In addition to antibiotics, commercial apple growers can use a growth regulator called Apogee.   Apogee should be applied in the spring when shoots are 1 to 3 inches long, and again a few weeks later.  Apogee reduces succulent shoot growth and there have been reports that it also stimulates natural defenses in the tree but the mode of action remains unclear.  (Kennelly)

The Apogee label is available here:  http://www.greenbook.net/docs/Label/L50185.PDF
 

Plant Disease Highlights  May 29 - June 2

   Apple/pear: Fire blight
   Apple/crabapple: Apple scab, Cedar-apple rust leaf spot, cedar-apple rust hypersensitive rust reaction (not frog-eye leaf spot)
   Hawthorne: Cedar-hawthorne rust, cedar-quince rust
   Honeylocust: Thyronectria canker, Honeylocust plant bug damage
   Hosta: Hosta virus X still being picked up by KDA
   Juniper: Botryosphaeria canker, old mite damage, winter dessication (to the tips)
   Maple: Anthracnose
   Pine: Winter dessication (to the tips), still seeing Dothistroma needle blight,  a fair number of Austrian pine trees dying but negative for pine wilt
   Rose: Heat scorch, powdery mildew, black spot, rose mosaic
   Spruce: Environmental needle scorch, Phytophthora root rot
   Sunflower: Bacterial leaf spot?
   Sycamore: Anthracnose + heat stress
   Wheat: Wheat streak mosaic virus

Contributing Authors:
Megan Kennelly 

kennelly@ksu.edu