Spring Checklist for Stone Fruit Disease Control

Tuesday, 15 April:

1. Brown rot: Remove last year’s brown rot mummies from trees:  Even though pruning on stone fruits is often delayed until after bud break (especially on peaches and nectarines), it is important to walk through stone fruit blocks and remove any brown rot mummies that remain attached to or suspended in the branches because these mummies will supply an abundance of inoculum for brown rot blossom blight if they are still present when trees begin to bloom.  Controlling blossom blight can be very difficult if mummies are present and warm rains occur during bloom.  If the brown rot mummies are removed during winter or early spring, they can usually be left in the sodded row middles between trees where they will rapidly break down before bloom.  However, when brown rot mummies are removed just before bloom, they should be collected and removed from the orchard to ensure that they can’t produce inoculum that will be blown into trees.
2. Black knot: Scout plum and tart cherry blocks and perimeter hedgerows for black knot. Prune out all of the knots and remove them from the vicinity of the orchard.  Black knot infections in the trunk or major limbs should be carved out with either a sharp knife or by shaving them out with a chainsaw.  The black, knot-like structures must be removed from the orchard because they will release massive numbers of ascospores beginning when trees are near white bud.  One or two knots left in a plum orchard can produce so many spores that even the best fungicide program will fail to prevent new infections. Black knots growing on wild Prunus in hedgerows can also generate inoculum for orchards and should therefore be removed.
3. Peach leaf curl:  Peach or nectarine blocks that did not receive a leaf curl fungicide at leaf-drop last fall should be protected with a leaf-curl spray at this spring. Copper, chlorothalonil (Bravo and generics), and ferbam are preferred materials, but only copper will provide the added benefit of suppressing the bacteria that cause bacterial spot.  Leaf curl sprays are especially important on peach/nectarine trees that had no crop last year because the fungicides applied during summer to control brown rot also help to prevent establishment of the leaf curl fungus. Blocks with no fruit generally do not receive fungicide sprays during summer, so leaf curl can be especially severe in those blocks the following year if leaf curl sprays are omitted.  Although leaf curl sprays in spring should be applied at bud swell, adequate suppression of leaf curl is often possible even if the leaf curl spray is applied after bud swell.  However, be aware that copper applied after green tissue is evident can severely damage the exposed tissue.
4. Bacterial canker in sweet cherries is best suppressed with copper sprays applied at leaf drop in autumn, but applications at dormant or bud swell in spring may also be beneficial.  If buds are already swollen, then use the lower label rates of copper so as to avoid the possibility that high levels of copper residues will be redistributed to flowers that will be opening in several weeks.
5. Apricots should be given a dormant spray of copper at high label rates to suppress both bacterial spot and bacterial canker.  No antibiotic sprays are registered to suppress bacterial spot on apricots, so a spring copper is especially important on this crop to suppress inoculum levels.  The bacterial canker pathogen can become systemic in apricots and will  kill young trees and major scaffold limbs on older trees.  To minimize the opportunity for bacterial canker to gain entry, I suggest that apricots should never be pruned in spring.  Instead prune apricots on hot dry days immediately after bloom (i.e., as the first step in crop load adjustment) and again after harvest.  The pathogen that causes bacterial canker (Pseudomonas syringae) is a cool weather pathogen whose populations tend to collapse when temperatures warm into the 80’s.
6. Be careful in siting new orchards: When planting new peach and nectarine trees, remember that most cultivars developed in California are extremely susceptible to bacterial spot.  Planting these super-susceptible cultivars next to other more resistant cultivars may put all of your peach/nectarine/apricot/plum plantings at risk of damage from bacterial spot because the super-susceptible cultivars will harbor inoculum that will then spread to other more resistant crops and cultivars.  If you can’t resist planting California varieties, at least plant them on a different part of your farm away from other stone fruits where they will be less likely to act as a “typhoid Mary” for the rest of your existing stone fruit plantings.