June 5, 2006 Volume 15 No. 12 Update on Pest Management and Crop Development
Geneva: Dogwood Borer 1st trap catch in Geneva and Wolcott.
San Jose Scale
Spotted Tentiform Leafminer
Potato leafhopper (PLH) is generally a more serious problem in the Hudson Valley than in western N.Y. or the Champlain Valley; however, the recent weather fronts have resulted in a sprinkling of reports in areas that are not always affected, so it doesn't hurt to tour observantly through a few orchards now. PLH does not overwinter in the northeast but instead migrates on thermals (warm air masses) from the south. Because PLH come in constantly during the season, there are no distinct broods or generations and the pest may be present continuously in orchards from June through harvest.
PLH feeds on tender young terminal leaves. Initially, injured leaves turn yellow around the edges, then become chlorotic and deformed (cupping upward) and later turn brown or scorched. Damage is caused by a toxin injected by PLH while feeding. PLH also occasionally causes symptoms similar to the effects of growth regulators, such as excessive branching preceding or beyond the point of extensive feeding. PLH damage is often mistaken for injury caused by herbicides, nutrient deficiency, or overfertilization. PLH injury may not be serious on mature trees but can severely stunt the growth of young trees.
Nymphs and adults should be counted on 50-100 randomly selected terminal leaves in an orchard. Older trees should be sampled approximately every three weeks during the summer. Young trees should be sampled weekly through July. PLH nymphs are often described as moving sideways like crabs, whereas WALH generally move forward and back. No formal studies have been conducted in N.Y. to determine the economic injury level for PLH on apples, so we suggest a tentative threshold of an average of one PLH (nymph or adult) per leaf. Little is known about the natural enemies of PLH, but it is assumed that they cannot effectively prevent damage by this pest in commercial New York orchards.
Damage by this migratory pest is usually worse when it shows up early. PLH can cause significant damage to newly planted trees that are not yet established. When PLH, white apple leafhopper (WALH), rose leafhopper (RLH) and aphids are present, control measures are often warranted.
Field trials were conducted during 2000 in the Hudson Valley to evaluate reduced rates of Provado against all three species of leafhoppers. We applied Provado in combinations at a full rate (2 oz/100 gal) and a quarter rate (0.5 oz/100 gal), at varying intervals (3rd-5th cover). We monitored nymphs of PLH/WALH/RLH and leaf damage by PLH.
Because of Provado's translaminar activity, all rates and schedules produced excellent control of WALH/RLH nymphs (however, reduced rates will not control leafminer). Against PLH nymphs, the number of applications was shown to be more important than rate; i.e., better protection of new foliage. Considering the percentage of leaves with PLH damage, the number of applications again appeared to be more important than application rate.
Although data on aphids were not taken, we know that Provado is an excellent aphicide, and the same principle would hold as for PLH -- maintaining coverage of new growth is more important than rate. Moreover, reduced rates are likely to increase the survival of cecidomyiid and syrphid predators that are common and effective biological control agents.
Leaf spotting caused by phytotoxicity from pesticide sprayscan be confused with leaf spotting diseases caused by fungi. Phytotoxicity may result when pesticides are applied at inappropriate rates, under unusual environmental conditions, or in untested mixtures with other products. It is impossible to list all of the potential materials or mixtures that might cause phytotoxicity because no one can evaluate all of the combinations that fruit growers mix in a spray tank, or to duplicate all of the foliage and environmental conditions that occur in orchards. Some of the more common culprits of phytotoxicity are listed below.
Captan is a potent fungicide on leaf surfaces, but captan is phytotoxic when it moves inside leaves or fruit. Most growers know that captan, if applied shortly before or after an oil spray, can cause severe leaf spotting, especially on Delicious (Fig. 1). There is no set delay that can be used for separating captan sprays and oil sprays because leaf condition at the time of application, rates of the two products, and varietal susceptibility to captan make a simple answer impossible. Captan-oil leaf spotting occurs because oil acts as an emulsifier that enables captan to diffuse into leaf cells. Even in the absence of oil, captan penetrates leaves more easily when leaves have developed under extended periods of cloudy, cool weather, because sunlight and dry conditions are required to stimulate development of the cuticle layer that prevents captan from reaching leaf cells. As might be expected, leaf spotting caused by captan-oil interactions is also more severe and the period of susceptibility is more extended when cloudy weather has limited cuticle development.
Captan-related leaf spotting can also occur when captan is tank-mixed with other products that are formulated with special wetting agents or penetrants. The captan label specifically states "The use of spreaders that cause excessive wetting is not advised."
Captan almost always causes some leaf spotting and/or shot-holing on captan-sensitive cultivars of sweet cherry and plum. The severity of the injury varies with the prior weather conditions and resulting leaf condition at the time of application. Leaf injury can be especially severe if captan is applied following cloudy, cool weather during a period of rapid shoot growth.
Over the past 20 years, I have seen cases of leaf spotting that have been traced to applications of various other pesticides, including Sevin XLR, Guthion, Lorsban, and Asana. In some cases, these products had been applied in mixtures with captan, whereas other cases involved mixtures with other pesticides. Most of these incidents did not result in serious leaf damage, and they are cited here only to illustrate that many different pesticides may cause phytotoxic leaf spotting under certain conditions.
In some cases, unusual sequences of pesticide combinations may contribute to phytotoxicity. Last week I visited an orchard with rather severe leaf spotting on mature Red Delicious trees (Fig. 2) where a tank-mix of Azinphos-methyl plus urea was applied in mid-May and was followed four days later with an application of Agrimek plus 1 gal of summer oil per acre. Adjacent Rome and Spartan trees showed very little injury, and no injury was evident in other orchard blocks that received the first spray of Azinphos-methyl plus urea but not the follow-up spray of Agrimek plus oil. I suspect that the urea softened the leaves enough to allow increased uptake of oil or of oil plus Azinphos-methyl residues when the second spray was applied 4 days after the first spray. Cool, cloudy conditions throughout mid-May was also a contributing factor.
As noted on the product label, Sovran can cause leaf spotting on some sweet cherry cultivars (Fig. 3). I have seen this damage on several farms where cherries were growing adjacent to apple trees that had been sprayed with Sovran.
The strobilurin fungicide azoxystrobin (Abound, Quadris, Heritage) is extremely phytotoxic to McIntosh, Gala, and some other apple cultivars. Drift from azoxystrobin applied to other crops can cause a leaf spotting on McIntosh that is indistinguishable from frog-eye leaf spot. Higher concentrations (as may result from residues left in a sprayer when switching from one crop to another) will cause extensive necrosis of leaf tissue (Fig. 4) and browning or russetting of the skin on apple fruit (Fig. 5). The large number of labeled uses for azoxystrobin raises the probability that apple growers in the northeast will experience occasional problems due to off-site drift of azoxystrobin. Azoxystrobin injury should be easy to diagnose because the leaf spotting will appear suddenly, will be evenly distributed throughout the canopy, and will occur only on McIntosh, Gala, and other Mac-related cultivars, whereas adjacent cultivars will be completely unaffected. The varietal susceptibility of apples to azoxystrobin injury is a useful distinguishing characteristic, because no other pesticide or fungal pathogen that might cause leaf spotting on apples would be similarly delimited by cultivar.
Gramoxone herbicide drifting onto apple leaves can cause a brilliant yellow leaf spot (Fig. 6), although the spots eventually turn brown and necrotic. Injury from herbicide drift is often more prevalent on low branches, but small spray droplets can drift throughout a tree canopy, sometimes causing an even distribution of leaf spotting that one might not associate with herbicide drift. The potential for foliage injury with gramoxone can be reduced by mixing a drift inhibitor with the herbicide. Drift inhibitors reduce the production of small spray droplets that are easily carried into the tree canopy by even the slightest breeze.
Summary: In commercial orchards that receive timely fungicide applications, most early season leaf spots are attributable to injury from agrichemical sprays. Risks of encountering phytotoxicity on leaves can usually be reduced by using proper sprayer calibration, following label restrictions on pesticide mixtures, and by keeping spray mixtures as simple as possible. The latter includes avoidance of untested mixtures of pesticides, micronutrients, and plant growth regulators, and avoidance of spray adjuvants not specifically required by either pesticide labels or unique water quality or other application conditions. Special care is required in years when the spring growth flush after bloom coincides with an extended period of cloudy, cool weather, because leaves that develop under those conditions are especially susceptible to injury by pesticide applications.
Fungicide rates in the Cornell Pest Management Guidelines for Tree Fruit are generally presented as rates per 100 gallons of dilute spray, although rates per acre are given for a few of the more recently registered materials. In the 1990s, we attempted to present most pesticide rates for tree fruit as rates per 100 gallons of dilute spray so that growers could easily plug those rates into their tree-row volume calculations. (The method for calculating tree row volume is explained at: http://www.nysaes.cornell.edu/ent/treefruit/html/2006TF03/2006TF03_23.php in the 2006 Cornell Pest Management Guidelines for Tree Fruit.) Where rates on fungicide product labels were presented as rates per acre, we traditionally divided those rates by 3 to arrive at a recommended rate per 100 gallons of dilute spray. In research trials where fungicides have been applied using a handgun, all fungicides have provided good disease control when one-third of the per-acre rate was mixed into 100 gallons of water and trees were sprayed to drip. Rates per 100 gal of dilute spray (i.e., trees sprayed to drip) should not be confused with rates per 100 gal of final mixture in an airblast sprayer tank because a concentration factor is also involved in calculating the latter.
Calculating appropriate fungicide rates gets more complicated when fungicides are applied in airblast sprayers because tree spacing, sprayer calibration, and nozzle arrangement can affect the proportion of the fungicide spray that actually lands in the tree canopy as compared to the proportion that lands in the ground cover beneath or between trees. To avoid selection for resistance, we have generally advised against using low rates of SI fungicides (Nova, Rubigan, Procure) even if trees are quite small. Thus, I would never suggest airblast applications of less than 4 oz/A of Nova, 8 fl oz/A of Rubigan, or 8 oz/A of Procure even for small apple trees. Rates for SI fungicides applied to control brown rot on stone fruits should also be kept within the rate/A range than is shown on product labels. Unfortunately, these stipulations for minimum rates per acre do not appear in the Cornell Guidelines. That will be corrected when the guidelines are revised for next year.
Determining minimum rates per acre (both for effectiveness and for resistance management) becomes complicated with newer fungicides such as Flint, Sovran, Pristine, and Scala because we have less data on effectiveness of low-rate airblast applications on small trees. Product labels designate a minimum rate/A for pome fruits for Sovran (3.2 oz/A) and Pristine (14.5 oz/A). Because these minimum rates are included on the federal labels, there is no legal option for reducing rates below those minimums. Having minimum rates/A for tree fruits specified on product labels can cause problems for growers using "smart sprayers" that automatically shut off nozzles to compensate for missing trees. A block with numerous missing trees might end up getting less than the designated minimum rate/A, and some growers have reportedly been cited by NY State DEC inspectors for such infractions. Nevertheless, when minimum rates are posted on labels, there is no doubt concerning what minimum rate should be recommended.
Based on what we currently know about selection for fungicide resistance, it would seem prudent to limit the minimum rates/A of Flint, Vangard, and Scala to the lowest rate/A that is listed on the product labels because all of these products are also subject to selection for resistance. Reducing those rates via tree-row volume calculations may work in some situations, but risks of control failures are increased. Selection for resistance will occur more rapidly if low rates of these products are used on a consistent basis.
Bottom line: With older protectant fungicides such as Captan, mancozeb fungicides, Polyram, sulfur, and copper fungicides, tree-row volume calculations can still be used to calculate effective doses for small trees. For newer fungicides, check fungicide labels for recommended rates/A and avoid using less than the minimum rate/A suggested on the product labels.