Apr 1, 2015
PSU: Learn from 2014 fire blight epidemic

For those of you who heard my fire blight presentation at the Mid­-Atlantic Fruit and Vegetable Convention and/or heard my talks during the 2015 winter meetings, you may be experiencing a sense of déjà vu… “haven’t we heard this before?”Well, you are correct. Motivational speaker Zig Ziglar said “repetition is the mother of learning, the father of action, which makes it the architect of accomplishment.”

Many people have a false sense of security thinking that since last year was so horrible, this year will be a cakewalk. That’s not how Mother Nature works. Regardless of what Mother Nature throws at us this year during bloom time, our goal is not relive 2014. So let’s review.

Don’t forget about those less obvious hosts everyone is aware apple, pear, and quince are targets for the fire blight bacteria. Don’t underestimate the hosts that linger in the orchard (crabapple), surrounding woods (hawthorn), and in the landscape (cotoneaster, fire thorn, mountain ash, Bradford pear).

The host range of the bacteria includes 130 species in 40 genera. Needless to say, this pathogen isn’t shy. These hosts are most likely not being controlled for the disease and, when conditions are favorable, are very susceptible to infection, providing additional sources of bacteria to wreak havoc in your orchards if these plants are nearby. In addition, not all apple cultivars and rootstocks are created equal when it comes to susceptibility to the disease. Remember: resistance does not mean immune.

If disease pressure is very high (such as in 2014), even traditionally tolerant cultivars, such as Red Delicious, can become infected. Where do the bacteria overwinter? The bacteria overwinter in living tissue surrounding cankers formed at the base of spurs or shoots killed the previous season. Cankers also will form where cuts were made to remove infected shoots during the growing season. Based on conversations the last couple of months, I know folks are seeing this in their orchards: growers who pruned infected areas out of trees during the season last year may be coming across old pruning cuts where the bark is split, darkened, and overall does not look “quite right.”Bacterial populations are influenced by temperature and can grow in a range of 50° F to 90° F.

Bacteria will begin to multiply at canker margins early spring, typically between tight cluster and early pink, and begin to ooze, and the ooze contains trillions of bacteria. This is important because this is when the bacteria are first available for dispersal in the orchard. Bacteria are on the move: Flowers Insects (mostly flies) are attracted to the sweet, sappy bacterial goo oozing from canker sites. These insects will begin to unknowingly disperse the bacteria from tree to tree by leaving colonies of bacteria wherever they walk. The bacteria are very good epiphytes, which means the bacteria will grow without causing disease. Consequently, they are capable of surviving and multiplying on plant surfaces several weeks before flowering begins.

In addition, the bacteria can also be dispersed by rain (directly or as aerosols), or carried on even modest winds. Once the first early opening flowers are colonized by bacteria, further dispersal is not only rapid, but specifically directed at open flowers through the activities of honey bees and other pollinators. The five stigmas in the center and top of each blossom have a moist, nutrient rich surface that supports their colonization of the bacteria to high levels even though infection has not yet occurred.

One day at 88˚ F is enough to build a very high bacterial population. As pollinators arrive to collect pollen, the bacteria are picked up on their body hairs and are then subsequently moved to other flowers in the orchard. Needless to say, it’s a vicious cycle. Warm weather builds the bacteria population; water/moisture triggers the infection event. Both bacteria and water must be present in the bloom for blossom infection to occur. If rain or dew does not occur during bloom, the pollinated flowers will go ahead and set healthy fruit despite the presence of the bacteria.

Unfortunately, bacteria have flexible legs call flagella, which make them very efficient swimmers, and if rain or dew does occur during flowering, the water will move the bacteria from the stigmas to the open nectaries, where over 90 percent of blossom infections occur. The nectaries are the open door for the bacteria to get inside the plant. Once inside the plant, the bacteria will move systemically via the plant’s vascular system.

The younger the tree, the faster the bacteria will move inside the tree. Above 60˚ F, colonization and infection of the nectaries appear to occur within minutes. Once a blossom infection event does occur, symptom development (black and wilted tissue) can range from 5­6 days under warm conditions to 30 days or more cool conditions, a fact that explains the puzzling appearance of blossom blight as much as a month after petal fall.

You may have heard the acronym “EIP”thrown around in the case of fire blight infection potential. EIP stands for Epiphytic Infection Potential. EIP is based on the assumption that a lot of bacteria are available in and around the orchard. As a general rule: If the EIP is less than 100 percent, few, if any, infections are likely to occur. An EIP of 100 ­150 percent is low, but is sufficient to support an epidemic of blossom blight. An EIP of more than 200 ­ 250 percent indicates that large numbers of infections are likely should a wetting event occur. It has been reported that NEWA will begin to include the EIP in their fire blight forecast.

How blossom blight can explode: 1. Initial bacteria are dispersed from cankers to trees up to several weeks before bloom. 2. Once flowers open and stigmas colonized, insects spread bacteria quickly. 3. Bacteria populations can double within 20 ­ 30 minutes. 4. One bacterium can lead to a trillion bacteria: each capable of initiating infection. 5. When all conditions for blossom blight exist, infections probably occur within minutes. 6. One infection event will lead to several hundred thousand infection events, making efficient control for the rest of the season difficult.

Canker blight is often a head scratcher and, consequently, grossly underestimated for its ability to cause damage in the orchard. Canker blight develops due to renewed activity by the bacteria at the margins of overwintering cankers from the previous season and occurs regularly every year where the disease is established. In other words, if cankers are left in your trees, you can count on canker blight.

The first symptom can be detected by cutting into the bark at the canker margin where you will see a narrow zone of water-­soaked green or diffuse brown tissue at the margin between the necrotic tissue of the canker and the surrounding healthy bark. The bacteria move systemically from the canker into nearby growing, succulent vegetative tissue. Often times, water sprouts close to active canker sites will develop a distinct yellow to orange color and begin to wilt. Canker blight symptoms are often overlooked because of their similarity to the more familiar shoot tip (shoot blight) infections that occur later. Another distinct feature is canker blight “shoot blight”will appear as if the infection is starting from the base of the shoot, as opposed to the shoot tip, which is characteristic of typical shoot blight.

In years when blossom infection events do not occur or have been well controlled, active canker sites serve as the primary source of bacteria for a continuing epidemic of secondary shoot blight infections that can lead to major limb, fruit and tree losses. Such sources of bacteria can also be important for new orchards with no history of fire blight when they occur in older, surrounding orchards from which the bacteria can be moved into young orchards by wind, blowing rain and certain insect species.

Shoot tip infections are incited on the youngest 2­3 tender, un­expanded leaves at the tips of vegetative shoots. The symptoms of these shoot tip infections differ from those of the systemically invaded shoots associated with canker blight in that the shoot tips are usually still green (not yellow to orange) when they wilt. The significance of these infections are twofold: 1) they tend to progress downward rapidly, often invading and destroying larger supporting limbs; and 2) as bacteria becomes abundant in the orchard, leaf surfaces are colonized by the bacteria (arriving from earlier blossom infections, active cankers or young shoots systemically invaded by bacteria from nearby cankers), but cause no harm so long as they remain on the surface and there is no injury.

Unfortunately, injury can easily occur. When potato leafhoppers feed on shoot tips, they will cause damage, thereby creating an entry for the bacteria to enter the plant. A more likely factor for injury is wind, and it does not necessarily need to be high winds associated with storms. Research has shown clear evidence that the simple damage to leaf hairs along the midrib of pear leaves provides suitable wounds for the bacteria to enter and to incite infections leading to typical shoot blight symptoms. Thus, the greatest number of shoot tip infections can occur during days with gusty winds that might cause a whipping type injury to shoot tip leaves, an event that is, unfortunately, all too common in the temperate climates where apples and pears are grown.

The incidence of severe fire blight associated with damage caused by hail and high wind is well known by experience. Much like shoot blight, it appears that leaf surfaces already colonized by the bacteria are severely injured during hail and wind storms so that the bacteria have ready access to internal leaf tissues and the vascular system.

It is important to note that trauma blight is not limited to highly susceptible cultivars, but can also occur in more resistant orchards, such as Red Delicious, since the injuries seem to breech the normal defense mechanisms active in these trees. When such trauma-­inducing events occur, the amount of fire blight that follows appears to be directly related to the amount of foliar colonization by the bacteria in the orchard, being heaviest near good sources of bacteria such as active blossom, canker or shoot blight symptoms or active cankers not previously removed.

While researching information about fire blight, I came across a good story illustrating trauma blight and the importance of removing cankers: A Maryland grower, who had followed a rigorous fire management program in his apple and pear orchards for several years, experienced a severe hail storm that struck his entire planting. The trauma blight symptoms that subsequently developed, however, were limited to less than 20 trees scattered in small clusters around the orchard. In nearly all cases close examination revealed the presence of an overlooked active canker. Because of his previous good efforts at limiting the number and distribution of these primary sources of bacteria, this grower was able to cut out and remove all of the blighted wood in his orchards within a few hours after symptoms were discovered.

Rootstock blight can be especially damaging where M.26 and M.9 apple rootstocks are used for high-density plantings. Bacteria from a single shoot infection can move rapidly down through the otherwise healthy superstructure of branches, limbs and trunk into the rootstock where the bacteria initiate a canker that quickly expands to girdle the tree causing the death of the whole tree. Early fall red coloration of trees in late summer to early autumn is indicative of girdling. Additional trees may show symptoms of decline and die in the early spring. While not all trees showing infections in the scion ultimately succumb to rootstock blight, 5­10 percent tree loss per year for the first 5­6 years is not uncommon and can be as high as 2,0­40 percent or more in some orchards. Note, too, that such losses can occur even with relatively resistant apple varieties, such as ‘Red Delicious,’ where scion infections can occur in trauma blight situations.

Dormant pruning of blighted limbs, shoots and cankers must be done every year to reduce the number and distribution of bacterial sources in and around the orchard before the bacteria can be dispersed in the early spring. In addition, remove wild or neglected fruit trees and other susceptible host plants from fencerows and areas nearby.

Be mindful about fertility since excessive amounts of nitrogen make trees more susceptible. Why? A vigorously growing tree will have the nutrients and water pumping fast in the vascular system to grow the tree. What else could be in the vascular system with the fast moving water and nutrients? Bacteria. Remember: bacteria are great swimmers. Legume cover crops should be avoided since they have nodules containing good bacteria, which fix nitrogen. Also avoid tree stressors, such as poor nutrition, inadequate drainage and nematodes since tree stress results in a tree less capable of resisting the progress of infection.

Apply early copper sprays, which will reduce bacteria colonizing bark and bud surfaces. Aim to apply 2 lbs/A of metallic copper at green tip. Remember: blossom sprays protect only flowers that are open and only protect blossoms prior the infection event. Since blossoms do not open all at once, it is necessary to apply several sprays when infection conditions are frequent during bloom. It is important to be vigilant in monitoring weather conditions: average temperatures greater than 60˚ F and wetting events (rain, heavy dew).

Unfortunately, applying fungicides or plant growth regulators during bloom using high volumes of water can provide a wetting event necessary for infection when all other conditions for blossom blight are present. Options available to protect blossoms and considerations to keep in mind: Apply antibiotics as complete sprays and add an adjuvant or surfactant. Antibiotic sprays are most effective when they are applied the day before or the day after an infection event (within 24 hrs!). Streptomycin is still the best option since it kills the bacteria and has partial systemic activity. Note: the systemic activity does not persist like fungicides and you have about a 48-hour window.

Streptomycin still works in the Mid­Atlantic. Kasugamycin is new to the market this year. It is different from streptomycin in that it reduces bacterial growth and reproduction, rather than killing it directly. Research in Michigan has shown this product has helped regions where streptomycin resistance is a big problem. Oxytetracycline is an antibiotic that functions similarly to kasugamycin in reducing bacterial growth. There is a four-spray maximum when applying antibiotics and do not apply antibiotics after bloom. This is necessary for resistance management.

Please do not think that just because three antibiotics are available you are able to apply 12 antibiotic sprays. Not only is it expensive, it is unnecessary and generally not a good idea. Blossom Protect is a live yeast product that colonizes the flower and prevents the bad fire blight bacteria from entering the nectaries. Research on the West Coast indicates this is a very successful product for controlling fire blight. However, this product is not as effective for our conditions on the East Coast at the present time. This is most likely due to the natural flower microbial community, which seems to prevent good colonization of the Blossom Protect.

I tested this product last year during very high pressure conditions and only achieved less than 40 percent control, whereas I had 76 perxcent control using streptomycin. In addition, the strep treated trees had significantly few instances of shoot strikes compared to the trees treated with Blossom Protect. Research is currently underway in Michigan to see what measures can be taken to make Blossom Protect work better in our conditions.

Although applying copper at bloom will kill bacteria, copper can cause fruit russetting and should be used with caution. Be mindful of rattail bloom. All blossoms are susceptible to infection if the bacteria and conditions are present. What to do during post bloom through terminal bud set. As mentioned previously, do not spray antibiotics post petal fall. A hail event is the exception. When making the decision to apply an antibiotic spray after a hail event, take cultivar susceptibility, fire blight history, PHI, and the ability to spray within 24 hrs into consideration if the crop value justifies the cost.

Shoot blight will be limited by applying the plant growth regulator, Apogee. The effect of Apogee occurs 10 ­14 days after application and can be tank mixed with streptomycin. It is not a streptomycin replacement. Apply during late bloom when active shoot growth is 1 ­ 3 inches. Apogee will harden off shoots, which will make the shoots not susceptible to shoot blight.

Monitor your orchard regularly for infections if there were blossom blight conditions and prune as necessary: symptoms manifest five to ­30 days post infection and shoot blight infection risk continues until shoot growth ceases. Research the last two years out of Virginia Tech has shown promising results using a tank mixture of Cueva and Double Nickel to manage shoot blight. Research is ongoing; however, during a high-risk season (such as conditions experienced during 2014), consider cover sprays starting petal fall using Cueva 2 qts/A plus Double Nickel 1 qt/A.

This will not help if cankers remain in the trees and “shoot blight”occurs as a result of canker blight. This season, my program will be studying the effect of weekly cover sprays of Cueva plus Double Nickel starting at petal fall. Since insects can cause wounds, which are entry points for the bacteria, be sure to control piercing-­sucking insects, such as aphids, leafhoppers and pear psylla.

Do not cut out infections during wet weather since bacteria move via water. Cut out active infections early ­ before necrosis develops (limits the spread of bacteria). Pruning is most effective when incidence is low. Focus on salvaging tree structure and young high-density plantings when incidence is high. Avoid excessive cutting since this stimulates secondary shoot growth. Bacteria can invade healthy tissue up to three feet in advance of visible symptoms, which makes tool sterilization not effective.

Practice the ugly stub method: cut 6­-12 inches below the margin of visible infection and remove later during winter pruning. Bacteria can live very well outside the plant and, to be certain you are getting rid of all sources of bacteria, it best to burn infected tissue that has been removed from the tree. Other considerations to keep in mind Fire blight can be confused with Nectria twig blight, which can also occur in late May ­ June and may have a shepherd’s crook like fire blight.

When you see shoot blight early in the season, closely examine the shoot to figure out if it is Nectria twig blight or fire blight. Nectria twig blight necrosis begins and the base of the shoot, whereas fire blight necrosis will begin at the shoot tip. Another diagnostic characteristic is Nectria twig blight will have orange colored spores present at the base of the shoot.

Kari Peter, Penn State University





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