A Few Points on Strep Use and Options for Using Apogee/Kudos in Shoot Blight and Canker Management on Apple
1. Blossom Blight Management – Follow Up on Recent Specific Questions
Apply streptomycin with Regulaid or LI700 just before or within 24 h after the wetting event that is predicted by RIMpro and/or Maryblyt (Infection Potential EIP value in NEWA) to allow fire blight infection. Repeat streptomycin spray/s during bloom after each subsequent and predicted rain event (just before or within 24 h after rain event) that indicates fire blight infection based on fire blight model. Streptomycin protects only blossoms that are open at the time of spraying. Once the spray deposit dries it will not redistribute with re-wetting. To protect newly opened flowers, repeated sprays are necessary if wetting events are predicted to occur during EIP 100 or above based on NEWA EIP fire blight model. Use NEWA’s great option to enter the “Streptomycin Spray Date”, which is under “Wetness Events Table” in NEWA fire blight model, and thus calculate the need for additional streptomycin applications using the NEWA’s fire blight models. It is especially important that you do not use Regulaid or LI700 with any sprays that contain thinning materials, so strep included with a petal fall spray should never be mixed with Regulaid/LI700. Adding to streptomycin a good adjuvant (and Regulaid is the best) will improve uptake of the streptomycin into surface tissues and that is especially beneficial when sprays are applied under fast-drying conditions. However, Regulaid should never be combined with captan, and other adjuvants that are sometimes used with captan that are generally applied at rates that will not provide the penetration effects like that which we get with Regulaid/LI700.
Pay special attention to timely applying streptomycin on late bloom varieties (fresh and cider) and in young apple orchards 1 to 8 years old. Young trees naturally bloom later and are prone to lingering bloom. All flowers opened late in the season are extremely prone to fire blight infections since fire blight ooze is most of the times readily available from fire blight cankers at this time of the year and will spread infections to the flowers.
In high density plantings, use of tall spindle or super tall spindle training systems on dwarfing rootstocks that produce smaller tree and are prone to suckering, there is almost no room for mistake in fire blight management. This is because if infection happens it can quickly progress through short-length branches into the trunk i.e. central leader and/or rootstock and the resulting fire blight cankers kill the tree by girdling.
2. Shoot Blight Management – Apogee/Kudos and Low Rate Copper
At petal fall, and this is a must if you had recent fire blight history (2 – 3 years in the past), apply Apogee or Kudos (prohexadione-calcium) when you have shoots 1-3 inches long (mid to late bloom) without concern for blossom blight pressure because Apogee is used to protect from shoot blight stage of this disease which can occur by direct infections from overwintered fire blight cankers. There will be leftover cankers because the active ones will not be visible to you. Yes, you can get infection of shoots by bacterium spreading from few infected flowers, and many times this is a prevalent way how shoot blight follows after flower infections. However, shoot blight can happen even after you had successfully controlled infections on flowers – and that is by a direct infections of shoots from carryover fire blight cankers from last year. Until terminal bud set on shoots is complete, shoots are susceptible to fire blight infections. Terminal bud set on shoots is a stage when the current year vegetative growth stops and a bud is formed at the end of the shoot. In an unusual weather conditions such as prolonged drought periods during spring and summer, additional flushes of growth can be triggered on shoots after terminal bud set has already occurred. These flushes are usually triggered by belated summer or fall rains which improve the water status of trees. In a year with normal weather patterns, terminal bud set occurs during June, varying somewhat among different apple cultivars. Apogee/Kudos inhibits gibberellin biosynthesis, which reduces longitudinal apple shoot growth and increases the thickness of cell walls in shoots thus preventing invasion of shoot tissues by fire blight bacterium and reducing shoot susceptibility to fire blight. Preventive spray applications start at 1- to 3-inch shoot growth stage and continue at 1- to 4-week intervals after the first application, to a total 3 applications. This use can reduce shoot blight incidence as much as 83% and shoot blight severity as much as 86%. Do not apply Apogee on ‘Empire’, ‘Stayman,’ or ‘Winesap’ apple trees as it can cause fruit cracking.
Apogee/Kudos shortens the period of terminal shoot growth. Because the trees are no longer susceptible to fire blight infections when shoots stop its growth i.e. when terminal buds set on shoots, application of Apogee will speed onset of terminal bud formation, thus reducing the period of shoot susceptibility to infection and reducing the chance for spread of fire blight on other shoots.For more information on terminal bud set please read: When and How Terminal Bud Set Occurs on Apples and Reduces Susceptibility to Fire Blight?
Shoot blight management should also include spraying a softer forms of copper materials (e.g. Cueva) at 0.2 lb/A of metallic copper equivalent (if slow drying conditions were highly likely to occur after this application, as even this low dose could cause fruit russetting at slow drying conditions). Copper will only kill fire blight bacteria present on the surface of shoots.
3. Tree Rescue with Apogee/Kudos of How to Prevent Formation of Cankers on Wood
If you know your trees had recently gone through a fire blight infection and you know you did not apply streptomycin on time, you can apply a rescue treatment with Apogee/Kudos in mature bearing orchards as a single application of 12 oz/100 gal. Window for applying this high rate is 2 to 3 days after an infection event on flowers that was not protected against with streptomycin. This high rate will stop or slow down shoot blight severity (lesion progress on shoots) and thus stop the invasion of wood by fire blight bacterium via the infected shoots. The sooner you apply this after infection event but before fire blight symptoms are visible, you will more successfully prevent development of fire blight cankers on the wood of your trees and thus reduce chances for tree death from fire blight cankers on limbs and central leader or rootstock. This use will also protect the other not-yet-infected shoots. There are two key reasons why we recommend this Apogee/Kudos rescue treatment 2 to 3 days after an infection event on flowers, that were not protected by streptomycin: (1) There is still chance to protect your tees from dying by fire blight cankers on trunk and you will easily prune out blighted flowers on thin spurs, unlike large cankers on trunk and limbs that will develop from infected shoots; (2) Apple flowering partially overlaps with shoot growth, and in most years, when severe fire blight epidemics were reported, conducive weather conditions occurred at late bloom through petal fall, when shoots intensively grow during bloom and petal fall (April). Shoot growth starts when King bloom flowers begin to open, which means that simultaneous infections of flowers and shoots by fire blight can and has happened many times before. In case you did not apply streptomycin to protect flowers on time, your priority has to shift to protect trees from dying by fire blight cankers – and you can do that by applying Apogee/Kudos at 12 oz/100 gal to prevent fire blight invasion of shoots that will lead to cankers on wood. For more information, please read this article: Effective Post-Infection Programs of Prohexadione-calcium for Reducing Shoot Blight and Preventing Fire Blight Canker Initiation on Apple Wood with Cost-Benefit Analysis.