June 12, 2006 Volume 15 No. 13 Update on Pest Management and Crop Development
Geneva: 1st Pandemis Leafroller
trap catch 6/8.
San Jose Scale
Spotted Tentiform Leafminer
Insect model degree day accumulations (as of June 13):
Apple growers will soon be confronting the issue of potential problems from internal worms in some blocks, and while some have resorted to preventive calendar sprays with or without mating disruption applied over the top, we continue to look at strategies that attempt to strike a balance between reasonable cost and acceptable risk in terms of which potential tactics to use.
In 2005, oriental fruit moth (OFM) management programs were tested in five "moderate-risk" commercial orchards, using three different pheromone dispensing technologies, as well as a modified fruit sampling procedure to assess the need and timing for special pesticide sprays directed against the 2nd and subsequent generation of this species. The pheromone treatments used were:
1) Isomate-M plastic ties, at 100/A
2) MSTRS OFM high-yield, low-density plastic pheromone packets, at 5-8/A
3) Hercon Disrupt Micro-Flake OFM, a sprayable plastic laminate
Treatments 1 and 2 were applied between 17-21 June; Treatment 3 between 7-8 July. In five additional "moderate-risk" commercial orchards, growers applied their conventional pest management programs (including one orchard where pheromones were used), and an on-tree fruit sampling procedure was used to determine the need for specific additional internal worm treatments based on the occurrence of new fruit feeding. In all cases, growers managed the first generation of OFM with their conventional pesticide applications that were directed primarily against plum curculio and obliquebanded leafroller occurring at and immediately following petal fall.
Pheromone treatment efficacy in depressing adult male trap catch was monitored using Pherocon IIB traps that were checked weekly from 9 May to 29 August. The fruit sampling protocol consisted of 8 weekly on-tree fruit inspections conducted from mid-July through August, comprising 300 fruits per plot during the first week and 100 fruits per plot on subsequent weeks, for each of the 2nd and 3rd generations, to detect the initial occurrence of any OFM larval fruit damage in time to curtail further infestation. Whenever an inspection session resulted in detection of at least one damaged fruit, the grower was notified so that he could determine whether a special spray of a selective pesticide was needed for control of internal Lepidoptera.
Trap catches of OFM were generally suppressed to low levels in all pheromone treatment plots during the mid- and late summer, although some breakthrough captures did occur, so trap shutdown was not absolute in all cases. Two plots at one Niagara Co. farm with notable OFM catches were located near a non-disrupted organic apple planting with a high OFM population, so it is possible that immigration from that block was too severe to be completely disrupted by the pheromone treatments in our plots. This site also recorded a relatively high CM catch during the first flight in June. Trap catches in the insecticide-treated plots differed among the sites, with some farms at or near zero for both species all season, and others catching relatively large moth numbers. Interestingly, if the proposed trap catch thresholds of 10 OFM and 5 CM/trap/week had been used as a basis for making control sprays, our management recommendations would have been much more conservative than they were using the evidence of fruit-feeding damage.
The fruit sampling procedure was convenient to implement, requiring 10–15 min per plot, and seemed to effectively allow detection of low-level infestations at a very early stage, so that the growers could be notified of any extra needed control measures in a timely fashion. Incidence of fruit injury was extremely low except for one (organic) site, which was the only pheromone plot location where more than 1 damaged fruit was detected per sampling bout. No damaged fruits were found during the 8 weekly samples in any of the Isomate plots; damage was detected 3 times each in any MSTRS and Hercon flake plots plots. In the insecticide-treated plots, damaged fruit was found during only one of these sessions, near the end of August. The high incidence of in-season fruit damage seen at the organic site was a result of high endemic pressure, of mostly OFM, plus the organic management regimen, which consisted of kaolin clay and B.t. for the 1st generation, and Cyd-X (codling moth granulosis virus) applied 4 times against the summer generations (20 July, at 5 oz/A; and 4, 20, 31 Aug, at 3 oz/A).
Our grower notifications of fruit damage during the season did not always result in a decision to apply an extra spray for internal worm control. Fruit damage caused by internal-feeding Lepidoptera at harvest was very low in all treatments at 3 of the 5 pheromone disruption sites. At the Niagara Co. site, the Isomate plot sustained approximately 10% fruit damage, although its proximity to a nondisrupted organic planting with a high population could have been a contributing factor. Additionally, the 1st generation CM we detected was not being disrupted, and damage from this species was likely included in the harvest evaluation, as no effort was made to distinguish between OFM and CM damage. The organic site had previously suffered relatively high fruit damage the previous season. Damage in all the treatments here ranged from 7–17% damage, which the grower indicated was acceptable for the organic processing market, and a measurable improvement over the previous season. No appreciable internal Lepidoptera feeding damage occurred in any of the 5 insecticide-treated plots.
Although the pheromone treatments tested were generally a useful component of the OFM management programs in these orchards, some factors can be identified as potentially contributing to less than perfect fruit quality:
- Plot size was not large enough to overcome the possibility of immigration by mated females;
- OFM population pressure was sometimes too high to be effectively disrupted by the pheromone treatments;
- the pheromones were applied only against 2nd and subsequent generations, leaving the potential for the 1st generation to contribute to fruit damage.
This research is being repeated and expanded this season, to continue to test the effectiveness of these application dispensers and the reliability of the fruit damage inspections. Nevertheless, it may be difficult to convince growers that any level of internal Lep damage before deciding to apply a treatment is acceptable in their orchards. (Full details and graphics available in this online report.
The most severe losses from fire blight occur when epidemics are initiated during bloom. The blossom blight phase of fire blight has been studied extensively. Cougar Blight, MaryBlyt, and other models have been used for predicting outbreaks of blossom blight and/or for timing streptomycin sprays. When applied at proper timings, streptomycin is very effective for controlling blossom blight. However, a few infections may escape even when strep sprays are properly timed.
The spread of fire blight after bloom (shoot blight phase) is less studied and less well understood than the blossom blight phase. Shoot blight sometimes results in significant loss of tree canopy and extensive loss of trees due to subsequent infections of susceptible rootstocks, even in orchards that had relatively little blossom blight. Even when shoot blight causes only minor direct damage, shoots that become infected during summer can result in cankers that carry inoculum to the next season, thereby creating increased potential for severe blossom blight the following year. Applications of streptomycin during summer are specifically NOT recommended, except immediately following hail storms, because regions that utilized summer sprays of streptomycin to control shoot blight have, without exception, developed strains of the fire blight bacterium, Erwinia amylovora (EA), that are resistant to streptomycin. Removal of infected shoots by pruning has advantages, disadvantages, and practical limitations that have been discussed in prior years: Scaffolds Fruit Journal 2004 and 1997
What contributes to the rapid spread of fire blight during summer? So far, no one has been able to predict when and why these outbreaks occur. Better control of shoot blight is a component of fire blight that requires more research. However, both the literature and field observations provide some interesting "clues" about factors that may contribute to summer spread of fire blight and possible options for reducing secondary spread.
Sucking insects have long been suspected of vectoring EA (i.e., carrying EA from tree to tree) or facilitating infection (i.e., generating feeding injuries that might allow wind and rain-dispersed EA to enter leaf tissue). McManus and Jones (1994) showed that shoot blight infections can be correlated with wind events that may cause leaf injury via wind whipping, but presence of insects that puncture epidermal tissue might facilitate infection in the absence of wind events.
The predominant sucking insects present on terminals in early summer in apple orchards are aphids and leafhoppers. The role of aphids has been evaluated in two studies, and both reported that aphids were incapable of vectoring or facilitating spread of fire blight (Plurad et al., 1967, Clarke et al., 1992). Pfeiffer et al. (1999), using caged insects, showed white apple leafhoppers caused no increase in fire blight incidence or severity.
Potato leafhoppers (Empoasca fabae, formerly E. mali), were implicated in some of the earliest studies of potential insect vectors/facilitators of fire blight (Brooks, 1926; Burrill, 1915; Gossard and Walton, 1922; Miller, 1929; Stewart and Leonard, 1916). Unlike aphids and white apple leafhoppers, potato leafhoppers (PLH) feed primarily in the phloem and their feeding injury causes physiological changes in the host. In their recent study with caged insects, Pfeiffer et al. (1999) reported that PLH caused a highly significant increase in fire blight in two out of the three years they conducted trials. They postulated that PLH facilitated bacterial entry through feeding wounds. Dissemination of bacteria by leafhoppers moving from tree to tree was not examined.
None of the published studies have provided definitive evidence that PLH actually transmits EA from plant to plant, nor has anyone proposed a threshold level of PLH that may be required before these insects impact the incidence of shoot blight during summer. Nevertheless, given the tremendous losses that fire blight can cause if it spreads during summer, it may be prudent to apply insecticide treatments to control PLH in orchards that have active fire blight. Approaches for controlling PLH were outlined in last week's issue of Scaffolds.
Apart from controlling piercing-sucking insects with insecticides, few other options have been investigated as controls for shoot blight. Apogee is very effective for reducing the incidence of shoot blight, but it must be applied beginning during late bloom, and that is too early to know whether or not streptomycin sprays have provided complete control of fire blight.
Jim Eve, a private crop consultant, has suggested that regular applications of sulfur fungicide may suppress spread of fire blight during summer. After Jim shared his observations with me, I discovered that although sulfur has never been tested as a control for fire blight, it was widely used and recommended as a control for PLH on various crops in the first half of the 20th century (Delong, 1934; Menusan, 1938; Miller, 1942). Thus, any blight control observed with sulfur might derive from its effect on PLH rather than from direct toxicity to EA bacteria. Effectiveness of sulfur for suppressing PLH and/or EA in apples remains to be proven, but the possible interactions among PLH, EA, and sulfur sprays raises interesting researchable questions.
Conclusions: The literature contains sufficient data to implicate PLH in summer spread of fire blight, although details (PHL threshold levels, effectiveness of adults vs. nymphs, etc.) remain to be worked out. The proven approach for controlling PLH will involve traditional insecticides such as Provado (as outlined in last week's Scaffolds article). However, the possibility that sulfur applied to control powdery mildew might also assist in suppressing fire blight is a hypothesis worth testing in research trials. Pruning out fire blight strikes as they appear is still recommended whenever removal by pruning is feasible.
Brooks, A.N. 1926. Studies of the epidemiology and control of fire blight of apple. Phytopathology 16:665-696.
Burrill, A.C. 1915. Insect control important in checking fire blight. Phytopathology 5:343-347.
Clarke, G.G., Hickey, K.D., and Travis, J.W. 1992. Fire blight management: evaluation of the role of aphids in transmission of bacteria and development of a computerized management system for growers. Penna. Fruit News 72(2):30-33.
Delong, D.M. 1934. The relative value of Bordeaux mixture, sulphur and pyrethrum products in reducing populations of the potato leafhopper (Empoasca fabae Harris). J. Econ. Ent. 27:525-533.
Gossard, H. A., and Walton, R.C. 1922. Dissemination of fire Blight. Ohio Agric. Expt. Sta. Bull. 357:83-126.
McManus, P.S., and Jones, A.L. 1994. Role of wind-driven rain, aerosols and contaminated budwood in incidence and spatial pattern of fire blight in an apple nursery. Plant Dis. 78:1059-1066.
Menusan, H, Jr. 1938. Results of potato dusting experiments on organic soils. J. Econ. Ent. 31:259-262
Miller, L.I. 1942. Peanut leafspot and leafhopper control. Bull. Va. Agric. Exp. Sta. 338:21.
Miller, P.W. 1929. Studies of fire blight of apple in Wisconsin. Jour. Agr. Res. 39:579-621.
Pfeiffer, D.G., Killian, J.C., and Yoder, K.S. 1999. Clarifying the roles of white apple leafhopper and potato leafhopper (Homoptera: Cicadellidae) in fire blight transmission in apple. Journal of Entomological Science 34(3):314-321.
Plurad, S. B., Goodman, R. N., and Enns, W. R. 1967. Factors influencing the efficacy of Aphis pomi as a potential vector for Erwinia amylovora. Phytopathology 57:1060-1063.
Stewart, V.B., and Leonard, M.D. 1916. Further studies in the role of insects in the dissemination of fire blight bacteria. Phytopathology 6:152-158.
The NYS DEC has granted Baythroid XL (Bayer CropScience, EPA Reg. No. 264-840) a registration for use on all pome and stone fruits in New York State. The a.i. is beta-cyfluthrin, a synthetic pyrethroid with activity on a broad range of the major insect pest species in these fruit crops, including leafhoppers, internal leps, leafrollers, plum curculio, apple maggot, sawflies, true bugs, San Jose scale crawlers, American plum borer, black cherry aphid, and cherry fruit fly. Per-acre use rates vary from 1.4-2.8 ounces, depending on the pest. This product has an REI of 12 hours and a PHI of 7 days (check label for details).
[Actually, note that Baythroid 2 (EPA Reg. No. 264-745), which had already been registered in NYS, but not on tree fruits, ALSO was recently granted a new label that now allows its use on these crops. This formulation's a.i. is 'regular' cyfluthrin, which is only half as effective as the 'beta' isomer. Growers can therefore use the 'old' Baythroid 2 on tree fruits, IF they get the new label from their distributor that shows the changes.]