Scaffolds 01 index

(Dave Rosenberger,dar22@cornell.edu, Plant Pathology, Highland)

Selecting and scheduling fungicides for apples involves many considerations. No single program can be devised that is appropriate for all apple orchards. The information that follows may prove useful as apple growers and consultants plan for the coming apple spray season. Note that throughout the following discussion, I have assumed that SI fungicides are still fully effective for controlling apple scab and powdery mildew in the orchards under consideration. Many of the suggested strategies must be modified if SI-resistant strains predominate in the orchard. Strategies for orchards with SI-resistance will not be covered in this article.

Fungicide classes referenced in this article include –

1. Contact fungicides: copper sprays (for use at green tip), mancozeb fungicides (Dithane, Penncozeb), Polyram, captan. Polyram is not a mancozeb fungicide. However, for the sake of simplicity, any subsequent references to 'mancozeb' in this paper should be interpreted as including Dithane, Penncozeb, and Polyram.

2. SI scab fungicides: Rubigan, Nova, Procure

3. Strobilurin fungicides: Sovran, Flint

4. Benzimidazoles: Topsin M and Benlate.

Decision #1: Copper sprays?

Copper sprays are strongly recommended in orchards where fire blight was present in either of the previous two seasons. Sprays should be applied between silver tip and quarter-inch green. The objective of this copper application is to create a copper residue on the tree that will release copper ions during rains and reduce populations of the fire blight bacterium coming from overwintering cankers. Copper sprays are of questionable value in orchards that have not had fire blight during the past two years since these orchards would theoretically have neither blight cankers nor resident populations of the fire blight bacterium. However, growers with highly susceptible cultivars and rootstocks may still wish to apply copper as insurance against blight infections that may have gone unnoticed the previous year.

In some seasons, copper sprays can cause russetting on apple fruit. This usually occurs when sprays are applied after quarter-inch green and/or there is little or no rainfall between the time of application and the time when trees reach open cluster. If too much copper residue is still present at open cluster, then rains occurring after open cluster may redistribute the copper residue to the clusters and injure the tissue that will later form the apple fruit.

If copper sprays are applied after quarter-inch green, then rates should be reduced to the minimum label rate. Conversely, if the long-range weather forecast at the time of copper application suggests that heavy rainfall is expected within the next week, then the high end of the labeled rate should be used to increase the probability that some copper residue will still be present after the rain. In either case, copper rates per acre should be adjusted for tree-row volume to prevent overdosing small trees.

A copper spray, even at the low label rate, will provide scab protection equivalent to that provided by a mancozeb fungicide applied at 1 lb of formulated material per 100 gallons. Copper sprays will not act as scab eradicants.

Decision #2: Contact fungicide program vs. planned use of SI's or strobilurins at tight cluster

Excellent scab control can be achieved by using only mancozeb or captan sprays. Rates as low as 2—3 lb of formulated mancozeb fungicides/A or 2—3 lb of captan 50W/A can provide excellent scab control if the fungicides are applied just ahead of predicted rains. Higher rates are needed in very large and/or poorly pruned trees or when fungicides are applied on a weekly schedule irrespective of rain events.

Programs involving only protectant fungicides (e.g., copper, mancozeb, Polyram, captan) can be inexpensive, especially in dry years, but they are unforgiving because they offer no post-infection or anti-sporulant activity. As a result, lapses in spray coverage will almost always result in at least a few scabby fruit. Coverage lapses may occur due to stretched spray intervals, wash-off during heavy rains, or spraying in windy conditions. Furthermore, if summer weather remains cool and wet as it did in 1998 and 2000, then any scab that becomes established during April, May or early June will continue to spread during summer and will increase the potential for late-season fruit infections that develop into pinpoint scab during storage. In wet years, attempting to control scab with only contact fungicides can be both frustrating and expensive.

All growers should begin the season by using one or two applications of contact fungicides because there is usually no reason to pay for an expensive fungicide prior to tight cluster. Growers planning to use only contact fungicides through the entire scab season will need to be more conservative in covering ahead of predicted infection periods than growers who plan to use SI or strobilurin fungicides during the peak scab season. Those planning to switch to SI's or strobilurins can afford more risk because the SI and strobilurin fungicides will cover minor lapses in coverage that may occur with prebloom applications of contact fungicides.

Decision #3: Knowing when to switch to SI's or strobilurins (the "power" fungicides)

The key to minimizing scab control costs is knowing exactly when to switch from a mancozeb or captan program to the extra protection provided by SI's or strobilurins. In a dry year when contact fungicides can easily be applied ahead of infection periods, contact fungicides alone may suffice for all sprays up to petal fall, especially in blocks where mildew is a minimal problem. However, in years with heavy rains and extended wetting periods, using SI's or strobilurins beginning at tight cluster may be the most cost-effective approach.

Regardless of the initial strategy chosen, it is absolutely essential that the first SI or strobilurin spray be applied BEFORE there are any visible scab lesions in the orchard. Therefore, anytime that protection with contact fungicides becomes suspect, either an SI or strobilurin should be applied within 12 days of the infection period in question. For example, if heavy rains (more than 1.5 inches) at half-inch green remove mancozeb residues and continued wind and rain prevent re-spraying ahead of the next infection period, then the best option is an SI or strobilurin application within 72—96 hours, counting from the time that the mancozeb protection lapsed. Unfortunately, cold fronts usually follow lengthy rain periods, and those cold fronts frequently bring winds that prevent good spray coverage.

One must sometimes choose between getting an SI or strobilurin applied within 96 hours under poor spray conditions or waiting beyond 96 hr for good spray conditions. I would usually opt for the latter choice when facing that decision. During the prebloom period, SI's and strobilurins applied within 10—12 days of the infection period should provide near-perfect scab control if they are applied so as to achieve near-perfect coverage. In fact, in one of my field trials last year, scab control was better when these fungicides were applied 10—12 days after infection rather than 5—8 days after infection. This means that one need not rush out to apply SI or strobilurin fungicides immediately if one has already missed the 96-hr window of post-infection activity following an infection. I believe that it is better to wait for good spray conditions during the following week than to attempt spraying in less than optimal conditions. For missed infection periods during or after bloom, one cannot afford to delay applications more than 7—8 days because higher temperatures will result in a shorter scab incubation period.

The suggestion that scab can be arrested even 10—12 days after prebloom infection periods is not meant to imply that prebloom contact sprays can be routinely omitted. Over-dependence on the post-infection and pre-symptom activity of SI's and strobilurins almost certainly will result in both occasional control failures and in rapid selection for fungicide-resistant isolates. Instead, the post-infection and pre-symptom activity of SI's and strobilurins should be viewed as a safety net for situations where protection by contact fungicides might be compromised. Knowing that such a safety net exists allows one to take a few more risks in timing prebloom contact sprays. But those taking the risks must also recognize that the safety net will be fully effective only if the SI or strobilurin program is initiated before scab symptoms become visible. That latter point cannot be over-emphasized. Strobilurins and SI's are much less effective when applied after scab is already visible on leaves because the visible lesions raise inoculum levels to the point where complete control becomes improbable.

(Next week: Part II - Configuring SI and strobilurin sprays, timing sprays during bloom, and strategies from petal fall through mid-July)

(Bill Turechek, wwt3@nysaes.cornell.edu, Plant Pathology, Geneva

Several diseases of stone fruit require attention early in the season. This article will provide a brief overview and management options for those diseases of peaches, nectarines, cherries, and plums.

Peaches and Nectarines

One of the most potentially serious diseases of peaches and nectarines is peach leaf curl, caused by the fungus Taphrina deformans. As the name suggests, the pathogen infects leaves, causing them to curl, and leaving them quite deformed. Infected leaves produce spores (called ascospores) that can infect additional leaves as well as the fruit. Severely infected leaves eventually shrivel and fall to the ground. Infected fruit either drop prematurely or remain on the tree and develop blisters or wart-like deformities on their surfaces.

The pathogen overwinters as spores in protected bark crevices and around buds. Initial infections occur in the early spring just as the buds begin to swell. The pathogen infects young undeveloped tissue, so infections are typically more severe when cool conditions prevent rapid development of the foliage. Severe infections occur during wetting periods with temperatures between 50—70°F. Once the leaves are fully expanded they are resistant to infection.

This disease is often controlled with one well-timed fungicide application. Applications are most effective when applied either in the autumn when 90% of the leaves have fallen or in spring just before bud swell. Ferbam, Bravo, and Ziram are labeled for control of peach leaf curl. However, copper hydroxide or copper sulfate are also labeled and may be the preferred pesticide, especially where bacterial spot is a problem.

Bacterial spot is a disease that affects virtually all stone fruits, but is particularly damaging to peach, nectarines, and apricots. The disease is caused by the bacterium Xanthomonas arboricola (previously known as Xanthomonas campestris pv. pruni). Except for Long Island, the disease does not typically cause significant losses in New York. However, last year was a particularly severe year in western New York and, therefore, the potential exists for another damaging year if management is neglected.

Bacterial spot affects the fruit, foliage, and young woody growth (twigs). Severe foliar infections result in leaf drop, which may significantly reduce tree vigor and winter hardiness. Bacteria from leaf infections may move into the current year's twig growth, leading to canker formation and providing an overwintering site for the bacteria. On fruit, the bacteria cause unsightly, dark-brown lesions and/or blemishes. Lesions often become sunken and the skin of the fruit cracks, causing deep pits that leave the fruit unmarketable. The disease favors warm and wet weather and may particularly be a problem in orchards with a history of disease. Primary fruit and leaf infection occur as a result of frequent wetting events from full bloom to 4 weeks after shuck split. Wind-driven rain or debris can damage leaves and developing fruit, creating small wounds that the bacteria can enter, and significantly affecting the occurrence and severity of fruit and leaf infection. Disease does not develop under hot and dry conditions.

The most effective way to manage bacterial spot is to avoid planting varieties that are highly susceptible to this disease. Yet, this is not always practical when processors ask for certain varieties such as 'Babygold 5' that are particularly susceptible to bacterial spot. Maintaining proper fertility is essential as excessive growth or poor nutrition both increase a tree's susceptibility. However, chemical control is typically necessary to manage disease. If bacterial spot has not been a problem in your orchard, a dormant application of copper is recommended as a preventive spray. This application can be timed to coincide with your peach leaf curl spray. If bacterial spot is problem later in the season, apply oxytetracycline on a 7—10-day schedule from shuck split until 3 week before harvest.

Phytophthora root, crown, and collar rot is a problematic disease that is best managed in the pre-planting phase through proper site selection, providing supplemental drainage, and selecting resistant rootstock. Peach and cherry rootstock are much more susceptible to this disease than apple rootstocks. However, even the best laid plans are compromised by unexpected events. If the disease has been a problem, or exceptionally wet weather favors disease development, apply Ridomil Gold at the label rate to the soil beneath the tree canopy in enough water to assure good coverage. Young trees and newly planted trees are at the greatest risk because they can be girdled more quickly than older trees.

Another disease that deserves attention early in the season is X-disease, a problem that within New York State is mostly confined to the Hudson Valley. This disease is caused by a mycoplasma and infects many varieties of stone fruits. Chokecherry (Prunus virginiannae) is a natural host/reservoir for the pathogen. The disease is spread from infected chokecherry, wild sweet cherry seedlings, and cultivated sweet cherry by leafhoppers. The only effective control measure for this disease is the eradication of chokecherry and infected cherry trees within a 500-ft radius of all peach, nectarine, and cherry orchards. Eradication can be accomplished with chemical brush killers – but be extremely careful not to use them within your orchard. If brush killers were used in the previous season, scout those areas in the early spring for regrowth.

"Peach X-disease

Cherries

Bacterial canker poses a serious threat to sweet cherry production. Bacterial canker is caused by the bacteria Pseudomonas syringae pv. syringae and Pseudomonas syringae pv. morsprunorum. The bacteria overwinter in bark tissue at canker margins, in apparently healthy buds and/or systemically in the vascular system. In the spring, particularly when conditions are cool and wet, bacteria multiply and emerge from their overwintering sites and are disseminated throughout the orchard by wind and rain. Natural plant openings (i.e., stomata, nectaries, and lenticels) or wounds provide entry sites for bacteria. The bacteria typically enter tree limbs and the trunk through pruning wounds and/or sites of freeze injury in autumn and early winter.

Upon infection, the bacteria multiply profusely and induce disease symptoms and/or replenish resident populations. They may reach axillary buds by systemic spread from infected leaves through the petiole throughout the season. Cankers subsequently appear at the base of infected buds. Canker expansion slows during winter but resumes again in early spring. Blossom infection occasionally occurs in some orchards. Infected blossoms are not always killed and may lead to fruit infection.

Bacterial canker is difficult to control. Removal of wild Prunus species in hedgerows adjacent to sweet cherry orchards may help to reduce inoculum. Growers should prune cankered limbs well below visible canker, avoid pruning in early spring and fall when bacteria are most active, and sterilize pruning tools before pruning healthy trees. Cankers also provide an entry site for Leucostoma (Valsa or fungal) canker. The area around the bases of young trees should be kept free of weeds and trash. This helps keep the trunk and crown dry and potentially reduces the build-up of the bacteria.

Injury to the bark perhaps provides the best avenue for infection. Injury as a result of frost damage often leaves wounds that are particularly susceptible to bacterial invasion. To avoid winter damage, paint tree trunks with a bright white latex paint to reflect the winter sun and minimize temperature fluctuations at the bark surface. Establish orchards on sites that do not have sandy soils, poorly drained, waterlogged soils, or sites that may suffer extended periods of drought. In newly planted orchards, establish trees with wide crotch angles to prevent the formation of wounds in the crotches.

As is true for many bacterial diseases, there is a limited chemical arsenal available to battle bacterial canker of cherry. Fixed-copper compounds are recommended for disease control. Current label recommendations call for one autumn application "before heavy rains begin" and another at late dormant. A third application is also labeled for use shortly after harvest in orchards where disease is severe, but is discouraged in most situations due to the phytotoxic effects of copper.

Plums and Prunes

Black knot is a destructive disease on susceptible varieties of plums and prunes (and in recent years on cherry). The disease is caused by the fungus Apiosporina morbosa and attacks only the woody tissues of the tree. Black knot develops slowly. Initial infections typically occur on new shoot growth during periods of rainfall that exceed six hours when temperatures range from 55 to 75°F. Susceptible twigs may become infected shortly after bud break when shoots are elongating rapidly. The first symptoms are usually evident by autumn as an olive-green swelling of the young woody tissue. Black knot develops rapidly the following summer to form the characteristic dark and warty knot. Knots vary in length from as small as 1 inch to nearly a foot and may or may not completely encircle the branch. The vascular tissue becomes restricted in infected branches, ultimately leading to the death of the branch.

Managing the disease requires removal of wild plum, prune, and cherry seedlings along fence rows, woodlots, and the orchard perimeter. Wild hosts infected with black knot are easy to spot if scouting is done before bud break. Binoculars may be needed to scan high twigs and branches of black cherry trees (Prunus serotina) as these trees can grow to more than 50 ft. Black knots should be pruned from infected trees within the orchard during the dormant season. Pruning cuts should be made at least 3 to 4 inches below the margin of the knot and infected twigs and branches should be removed from the orchard and buried or burned. Fungicides should be applied from white bud through shuck split; however, chemical treatment is most likely to be effective only if pruning and sanitation has been practiced. The most effective fungicide for black knot control is Bravo. Bravo's use is permitted under FIFRA Section 2(ee) and has been approved by the NY-DEC (NY users must have a copy of the DEC approval at time of application). Bravo is not labeled on plums after shuck split.

Summary

• Dormant application of copper for peach leaf curl, bacterial spot of peach, and bacterial canker of cherry.

• Soil drench with Ridomil Gold in problem areas for Phytophthora crown, root, and collar rot.

• Prune out black knots, remove from orchard and destroy by burning or burying.

• Destroy wild plum, prune, cherry, and chokecherry in a 500 ft radius of orchard for black knot and/or X-disease control.

end of this issue

scaffolds index