The latest growers need to know for apple scab, fire blight, sooty blotch and flyspeck, rots, bacterial spot, and cherry leaf spot.
Quick question: Where did the month of June go? I don’t know about you, but this past month went lightning fast. In my experience, lightning fast months are usually attributed to one being exceptionally busy—so, thank you, Mother Nature, for not allowing me to have idle hands this month—or any month this year!
We have had a lot of mysteries in the orchard pop up since mid-May: mystery shoot dieback (not fire blight); mystery cankers on young trees; mystery tree decline. Needless to say, we’ve had to put on our Sherlock Holmes hat more times than not this season. We’re still trying to get to the bottom of some problems but the mysteries have been balanced out by typical season culprits we can usually count on year to year.
What needs to be on your radar as we dive officially into the summer season:
Apple Scab: Primary is over
Primary scab infection is defined as the ascospores released from the overwintered leaves from the previous year’s infected leaves. For the last two weeks, we have not detected any scab spore released from last year’s leaves indicating the primary infection period is over. If you made it through the primary infection period with no scab to be seen: Congratulations! However, it’s important to continue to scout your orchards since we had a few good infection periods during the down swing of the primary period.
Whatever spores established on leaves and/or fruit during the primary infection period will produce additional spores called conidia in vast quantities. These summer spores can cause infection throughout the summer and we call this stage the “secondary infection period.” It is important to monitor your orchard for any scab infection that became established during the “primary” period because scab control will be needed throughout summer in order to keep the disease from causing significant damage to fruit, especially if we continue to experience weather conditions ideal for infection.
Fire Blight: Worrying about shoot blight
It’s that time of year and shoot blight continues to be a headache for a lot of growers. The very warm temperatures, combined with leftover cankers that were not pruned out during the winter make ideal conditions for shoot blight incidence. Leftover cankers produce “canker blight,” which is another version of shoot blight. Let’s review the differences and play the broken record again:
The importance of cankers and canker blight
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 (the 2016 season), 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.
In contrast to characteristic “shoot blight”…
Shoot tip infections are incited on the youngest 2-3 tender, un-expanded 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:
- they tend to progress downward rapidly, often invading and destroying larger supporting limbs.
- 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.
Are there management options?
If you are experiencing "shoot blight" AND you know you have overwintering cankers in your trees, the only management option is to prune out the affected parts of the trees.
- There are no topical spray applications you can use to control this version of shoot blight: the infection is coming from inside the tree and not outside.
Once terminal bud set occurs, the tree stops growing and the bacteria stop moving within the tree. The shoot blight as a result of canker blight should stop. However, a lot of damage and establishment of more cankers can develop between now and terminal bud set. Folks have been told all along to prune out fire blight during the season when they see it; however, there can be too much of a good thing:
It is very important to avoid excessive cutting when pruning out fire blight. Excessive cutting will encourage more shoot growth and make your fire blight problem even worse.
This is especially true for older orchards where cankers may be lurking about in the tree. If you start to see a lot of shoot blight in older orchards in a few weeks, it is best to just leave the tree(s) be.
A healthy review of for how to cut out fire blight infections
- 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 ~3 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.
The stormy weather lately and fire blight
Typical of summer, the severe weather seems to be picking and choosing where to unleash its wrath. During the week of June 20, severe storms dumped very large hail in parts of Adams and York counties; word on the street, a tornado nailed central Maryland. Yikes. As many growers know, these types of weather events prove to be especially problematic when it comes to fire blight.
Why? Any shoot blight occurring in the orchard is oozing bacteria and bacteria can be blown around very well. Often times the bacteria will hang out on the leaves and other plant tissue, not causing any issue, until there is some kind of wound event (hail, high winds, etc.). Many times, crazy winds accompanying severe storms, even when hail or a tornado doesn’t result, can cause damage, whether you can see it or not. As far as managing the spread of the disease, which is considered “trauma blight,” growers are encouraged to monitor their orchards closely and prune infections as soon as possible.
If obvious damage occurs (hail, etc.), spray an antibiotic (streptomycin) within the first 24 hrs of the event to mitigate infections. In addition, there has been some success with slowing down this “external” shoot blight using Cueva 2 qts/A plus Double Nickel 1 qt/A (or Cueva by itself); anecdotally, some growers have used Serenade Optimum 1 lb/A, as well.
Sooty Blotch, Flyspeck, and Rots
Depending on your location, the management threshold for SBFS was reached anywhere from the beginning of the month until the last week of June. Since management for apple scab is recommended through second cover, folks should have been okay earlier in June since many of the chemicals include SBFS control. As we continue to forge ahead through the season, summer cover sprays including Captan, Topsin M, Ziram, and strobilurins (Flint, Sovran, Merivon, Luna Sensation) will provide excellent protection.
As we are in the midst of the season where storms can dump a lot of rain in a short period of time, remember that 2 inches of rain washes off any protection you may have applied.
Fruit rots typically manifest in July and August and fruit susceptibility increases as it begins to mature. The optimum conditions for disease development include rainfall, relative humidity of 80 to 100%, and warm temperatures (80–90 °F). Fruit rot disease management is most effective by applying fungicides on a 10- to 14-day interval schedule through harvest; more frequent under favorable conditions. Captan and ziram provide good protective control and is most optimal when combined with another fungicide. Strobilurins (Flint, Sovran, Merivon) are excellent for controlling fruit rot. To reduce the risk of resistance, alternating a fungicide with another FRAC code and tank mixing are encouraged.
There are a few alternative options available for managing summer diseases, in addition to sulfur. Last year, we saw excellent SBFS control using Oso (polyoxin D salt; Certis). For two seasons in a row, we used Serenade Optimum (Bayer) during the last two cover sprays and it showed promise managing fruit rots later in the season and even in cold storage (disclosure: we used a conventional program up until sixth cover). In addition, Cueva and Double Nickel (Certis) has also shown some control for summer disease management (we are evaluating this for the 2016 season). It is very important for those using alternative options to be vigilant about keeping up with management. This means closely monitoring the weather, as well as spraying more often when infection conditions persist.
Bacterial Spot: Favorable weather conditions lately
Conditions have been favorable lately for bacterial spot on peaches and nectarines and symptoms are afoot in the orchard on susceptible cultivars. For those who don’t have fruit this season on certain stone fruit varieties, this disease affects the leaves and management is needed. Shorter application intervals should be used when rainy periods are frequent and temperatures range from 75°F to 85°F. A longer 14-day interval is acceptable during extended periods of dry weather. Management options include oxytetracycline (MycoShield, FireLine; 21 d PHI) and copper. A list of copper options are available at the bottom of this Fruit Times article from April 1, 2016.
Coppers vary in their PHI, so use products accordingly when you are nearing harvest. When using copper, remember slow drying conditions and pH of the solution will influence copper injury. In addition: be mindful of what you tank mix with your copper (NO foliar fertilizers and beware of any product that will make the solution more acidic). Other controls to consider for incorporating into a management rotation are Serenade Optimum (Bayer) and Double Nickel (Certis).
Cherry Leaf Spot: Management needed postharvest
Cherry leaf spot is similar to apple scab when it comes to infection conditions: warm and wet. The cherry leaf spot fungus prefers moderately wet conditions (hours of leaf wetness), with temperatures above 60°F. Although folks may be at the end of their tart cherry harvest, disease management is still necessary postharvest. Growers are encouraged to apply two postharvest fungicide applications to prevent cherry leaf spot infection. The goal for the remainder of the season is to keep those leaves on the trees and prevent premature defoliation due to CLS infection. Tart cherry trees should not be bald in August. This will stress trees setting them up for a weakened condition as they enter the winter months. Many options are available, such as Bravo, Indar, Merivon, and Syllit (to name a few).