Note: This article was first published in Scaffolds Fruit Journal 29:1 (23 March 2020) but some of the hyperlinks in that article did not function correctly.
Attention to a few disease-control details in early spring can make life easier through the rest of the summer. Following is my list of reminders:
For apple scab and Marssonina: In blocks or around trees where these diseases were a problem last year, disease control this year will be much easier if over-wintering leaves can be removed, shredded with a mower, or sprayed with urea prior to bud break. The latter two approaches will speed decay of the leaves, thereby destroying the overwintering phases of the fungi before the full complement of spores can be released. Details for these processes have been described elsewhere (Rosenberger, 2005; Acimovic 2020). Where neither of these diseases were a problem last year, there is no reason to spend extra time and money on managing leaf litter.
For fire blight: Where fire blight was a problem last year, all diseased twigs, branches, and cankers should have been removed during winter pruning. The pathogen in winter prunings will lose viability as the prunings dry out. So far as we know, there is no risk of spreading the pathogen by shredding the prunings with a flail mower. After winter pruning to remove cankers, the next line of defense is provided by a copper spray applied at silver tip or green tip. The preponderance of evidence suggests that new low-rate coppers provide relatively short residual activity on trees (Rosenberger et al., 2013). For the silver tip spray, we generally suggest copper products that can be applied at rates that will result in application of at least 2 lb of elemental copper per acre, thereby providing more extended protection. In years with a late spring when trees are expected to progress rapidly from green tip to tight cluster, using lower rates of copper may help to reduce the risk of fruit russetting that can occur if copper residues persist too long and injure tissue at the base of flower buds that later develops into fruit. However, weather so far this year suggests that we may have a more drawn-out spring were higher rates of copper will prove beneficial for controlling fire blight in problem blocks. In blocks where no fire blight has been found in either of the past two years, there is little if any reason to apply copper. Copper is used to suppress bacteria emerging from obscure infections that remain in the orchard after pruning. If there has been no evidence of fire blight for two years, then presumably there are no active blight infections and therefore no targets for the copper sprays in apples or pears.
For bitter rot: In blocks were bitter rot was a problem last year, remove all fallen fruit, fruit mummies, and pruned twigs from beneath trees and either dispose of them away from the orchard or flail-chop them in row middles to break them down for more rapid decay. Rotted fruit left on the orchard floor over winter have been recognized as inoculum sources since 1903 (Schrenk and Spaulding, 1903; also see commentary at http://blogs.cornell.edu/plantpathhvl/apple-diseases/summer-diseases/bitter-rot/ ). Twigs pruned from trees last summer or this spring can be colonized by the bitter rot pathogens (Fig. 1). Those colonized twigs may produce inoculum for fruit decay in summer.
For crab apple pollenizor trees: Where crab apples are used to enhance cross pollination, any cankers in the crab apple trees should be removed during winter pruning. In Washington State, cankers on crab apple pollenizer trees have been identified as the inoculum sources for several important postharvest pathogens of apples and pears (Xiao et al., 2014). The diseases identified on the west coast are not very common as postharvest pathogens in eastern apple orchards, perhaps because those diseases are suppressed by the extensive summer fungicide programs that we must employ to protect fruit from flyspeck and other summer fruit rots. Nevertheless, it is unwise to allow fungal cankers to persist within orchards even if they are found only on pollenizer trees.
Black knot: For those growing plums and tart cherries, any black knots (Fig. 2) found in trees should be pruned out before trees reach the white bud stage by cutting at least 8 inches below the existing knot. The knots that are removed should be either burned immediately, buried, or bagged for disposal with other trash that is sent to a landfill. Do NOT dispose of the pruned-out knots in a hedgerow beside your orchard: Spores from the prunings will blow back into the orchard and cause new infections. (Even though I should have known this, I learned it the hard way in a research block many years ago!) Removing black knots from infected trees is the primary method for controlling this disease. Fungicides applied during bloom will NOT protect trees when there is an abundance of inoculum from knots that were not removed during winter.
Peach leaf curl: Peach leaf curl (Fig. 3) is relatively easy to control using copper sprays or one of the labeled fungicides that are listed in Cornell’s Pest Management Guidelines for Commercial Tree Fruit Production. Applications can be made during leaf drop in autumn or at bud swell in spring. If the peach crop was frozen out last year and no brown rot fungicides were applied to protect fruit last summer, then trees may be more prone to leaf curl this year. It seems that fungicides applied during summer to control brown rot reduce survival of leaf curl during summer and thereby reduce disease pressure for the following year. Summer fungicides alone are not enough: they must still be supplemented with either the leaf fall or the swollen bud application recommended for controlling peach leaf curl. Nevertheless, leaf curl disasters in the Hudson Valley have often been associated with orchards where the leaf curl spray was omitted or mis-timed in a year following a frozen-out crop.
Leaf curl sprays in spring should not be applied before buds are swollen because the leaf curl fungus overwinters in the buds and fungicides applied to fully dormant buds can be washed off before the buds swell enough to allow the fungicide to contact the overwintering fungus. Leaf curl sprays applied after buds have opened may be less than 100% effective, but applying a leaf curl spray a bit late is better than not applying any control at all.
For all of the above: “A stitch in time saves nine.” Stopping or slowing diseases before they get started is much easier and more cost-effective than trying to stop them after they begin to spread. For an apt illustration, check your local news feed for commentary on the coronavirus.
Rosenberger, D.A., Rugh, A.L., Feldman, P.M., and Truncali, D.N. 2013. Comparison of copper products applied at green hip to control fire blight on apples. Pages 7-10 in blogs.cornell.edu/dist/d/3767/files/2013/11/2013-HVL-Field-trial-data-compr-203ay9s.pdf
Rosenberger, D.A. 2005. Jump starting apple scab control programs in high-inoculum orchards. Scaffolds Fruit Journal 14(1): 3-5. http://www.scaffolds.entomology.cornell.edu/2005/050321.html#disease
Schrenk, H.V. and Spaulding, P. 1903. The bitter rot of apples. USDA Bureau of Plant Industries Bull. 41. 63 p. https:// blogs.cornell.edu/dist/d/3767/files/2017/02/Shrenk-and-Spaulding-1903b-Bitter-rot-of-apple-18k8awc.pdf
Xiao, C. L., Kim, Y. K., and Boal, R. J. 2014. Sources and availability of inoculum and seasonal survival of Sphaeropsis pyriputrescens in apple orchards. Plant Dis. 98:1043-1049. (Note: color photos in the middle of the article are useful: https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-12-13-1218-RE )