Connecticut Fruit Growers Newsletter
April 4, 1997
IN THIS ISSUE:
APPLE SCAB AT GREEN TIP
Where the unusually wet conditions last year produced more than the normal amount of leaf scab, it would be risky to delay the first fungicide to Half-Inch Green this year. The probability of early scab infection during scab infection periods is directly related to the number of scabby leaves on the orchard floor from last year. We know spore maturity has not been delayed. Dr. Donald Aylor found some scab spores had already matured at the Connecticut Agricultural Experiment Station in Hamden by the end of February.
The comments of Dr. David Rosenberger below, in a April 1 newsletter for New York orchardists, are applicable to Connecticut growers.
Remember that were you don't plan to use a sterol inhibitor fungicide before Tight Cluster, fungicide sprays should be applied before the predicted rainfall. After-infection efficacy for contact fungicides is not good for more than 24 hours, and orchard floors are prone to be soft after early-season rains. Benlate, Topsin M, and dodine are possible alternatives to sterol inhibitor fungicides for after-infection scab eradication if they have not been used more than 2-3 times per year in previous years. These materials have been ineffective against scab in some New York orchards having a history of frequent use.
PESTICIDES CAN DISSIPATE RIGHT IN THE SPRAY TANK
The following article by Dr. Art Agnello, Cornell, Geneva, is included to remind growers of the need to check acidity of their spray water.
PEACH LEAF CURL FUNGICIDE
Peach trees that did not get a fungicide treatment last Fall after leaf drop should be treated as soon as possible now, before buds can become infected by the leaf curl fungus. Ferbam, Ziram, Bravo, Kocide copper, or copper oxychloride sulfate or effective options. Peach leaf curl does not develop on unprotected peach trees every year, but only when weather conditions are favorable in March or April. Because symptom development cannot be prevented after infection has taken place, preventive fungicide treatment is the only sure control. I am not aware that any external secondary spread of early infection has ever occurred in New England, but published descriptions of the disease indicate secondary spread can continue through the growing season. This would surely weaken affected trees.
PEAR PSYLLA CONTROL OPTIONS
Pre-Bloom:
Petroleum spray oil 2%, applied dilute or 1/3 dilute volume to delay egg-laying. Psylla adults prefer to wait for appearance of green tissue than to oviposit on oily bark. If oil is applied before egg-laying has begun (usually about mid-March in Connecticut), one can prevent early appearance of the sucking nymphs. A 2% oil spray lasts 3-4 weeks, and must be renewed for continuing effect.
Petroleum spray oil, 1 2% applied dilute just before the White Bud stage of flower development. Psylla nymphs that are wetted by an oil spray are killed. Oil does not kill psylla eggs. By the Green Bud flower stage, many eggs will have hatched if oil was not applied earlier, but it is unlikely that the tougher to kill hardshell stage would appear by then. Whether ultra fine oil is less likely to have phytotoxic effects than standard spray oil is not known. Reports from Washington state indicate even ultra fine oil can produce some skin roughening on the Anjou variety when used in a series of post-bloom spray.
A good application of oil shortly before bloom should hold off appearance of any hardshell nymphs to at least Petal Fall. This delayed oil strategy will entail fewer oil sprays than the early oil method.
Morestan 25WP, 1 2 lbs per 100 gallons dilute basis, applied dilute or 1/3 dilute at the White Bud stage will kills eggs and young active stages of psylla and European red mite by direct spray contact and residual contact. Do not use Morestan within 10 days of an oil spray. Clean sprayer of oil before using for a Morestan application.
Early Post-Bloom:
A series of three oil sprays, 1 2% in dilute application, beginning at Petal Fall, and repeated at 10-14 day intervals. Each spray is meant to kill all psylla nymphs that have appeared at that time. If spray intervals are too long, some nymphs will become hardshells or adults that may escape oil kill. Post-bloom will not be a satisfactory option if pre-bloom measures have not been taken to preclude appearance of hardshells by Petal Fall. All viable eggs from the overwintered generation of pear psylla are expected to have hatched by 2 weeks after Petal Fall. If no nymphs are allowed to survive to adulthood, there will be no second generation.
Agri-Mek 0.15 EC plus spray oil (20 fl oz, plus 1 gallon, per acre). For best results, an Agri-Mek treatment should be preceded by oil or Morestan as described above in order preclude appearance of any hardshell nymphs or first generation adults before the Agri-Mek spraying. Then the Agri-Mek can be delayed until the overwintered adults have exhausted their egg supply, an event that occurs about a week after Petal Fall. Apply the spray soon after a week following Petal Fall in order to catch all leaves in a tender state that will absorb the Agri-Mek. Hardened, leathery leaves do not absorb the toxicant, no nymphs could survive on those leaves.
Pull unneeded shoots from interior scaffold and central leader positions. This eliminates preferred egg-laying sites that attract adult psylla. Shoots should be removed in June before they are difficult to remove by hand, and before any second generation nymphs have reached the hardshell stage. This practice is very useful whether or not previous psylla control measures have been used.
PEAR SCAB
Although this fungal disease resembles apple scab, it is caused by a different organism. Temperature and duration of wet conditions necessary for infection are quite similar to those for apple scab. However, scab on pears has not been a serious problem in some Connecticut orchards and some varieties. If no scab was present on fruits or leaves in 1996, there may be no need to begin fungicide sprays before Green Cluster (when flower buds are visible, but no white petal tissue shows), or White Bud stage, when Fabraea control should begin.
Fungicides labeled for pear include Carbamate, Ziram, mancozeb, Benlate, sulfur, and Procure. Rubigan is labeled for use only after bloom.
RISKS AND BENEFITS OF COPPER SPRAYS
(from Dr. David Rosenberger, in Scaffolds, March 25, 1996)
A copper spray applied at the Green Tip bud stage has been recommended for more than 40 years as a part of a fire blight control strategy for apples and pears. Copper sprays also have been used to reduce the incidence of a canker problem that is apparently associated with winter injury on the 'Marshall' strain of McIntosh. However, copper sprays can cause fruit russetting on apples and pears, so copper sprays should be used with caution. Researchers generally agree that the Green Tip copper spray reduces the amount of inoculum that is produced by overwintering blight cankers. However, a review of experiments performed by various researchers all over the U.S. reveals that the Green Tip copper spray has performed very inconsistently.
In some cases, the Green Tip spray had almost no effect on subsequent development of fire blight, whereas in other cases it was very effective. Variability may be related to specific weather conditions during the tests, the amounts of copper residues remaining in the trees several weeks
after application when blight cankers become active, and the sources of inoculum that contributed to the fire blight epidemics in the various tests. In some experiments, the effectiveness of copper sprays in small test plots may have been confounded by bees and other insects that could have carried inoculum from untreated plots into the treated plots during bloom. Despite variable results in experiments with copper sprays, the tree fruit specialists in New York believe that the Green Tip copper spray is a valuable tool for controlling fire blight. This spray is included in Cornell's "Pest Management Recommendations for Commercial Tree Fruit". The Green Tip copper spray is not a substitute for appropriate applications of streptomycin during bloom, but orchards sprayed with copper are likely to have less blight than comparable orchards where the copper spray is omitted.
When deciding whether or not to apply a Green Tip copper spray, growers must consider the fact that this copper spray has contributed to severe fruit russetting in some years. In 1990, up to 65% of fruit in several Hudson Valley orchards were out of grade because of russet. Where fruit
russetting has occurred, one or more of the following have been implicated as contributing factors:
Copper sprays were applied after Green Tip. As trees approach Half-Inch Green, the expanding buds trap and hold more copper. As a result, copper residues remaining in the tree at the Tight Cluster and Pink bud stages can be high enough to cause phytotoxocity.
Less than 2.5 inches of rain occurred between the copper applications and the time trees reached Tight Cluster. Copper residues were not adequately weathered by Tight Cluster.
Sprayers were not properly calibrated and application rates were too high.
Spring frosts occurred just before or after copper applications. As plant cells freeze and thaw during spring frosts, copper ions may get pulled into cells and cause more damage than they would in the absence of frosts.
To minimize the likelihood that copper sprays will cause fruit russetting, the following precautions are recommended:
1. Use copper sprays only on Marshall Macs (to prevent Marshall Mac canker) and on other apple and pear blocks that had fire blight during the previous 2-3 years. Copper helps control fire blight by reducing inoculum from overwintering cankers. There is no reason to use copper where blight has never been a problem (except in the case of newly planted trees as described below).
2. Apply copper sprays no later than Green Tip.
3. Calculate rates carefully using tree-row-volume calculations to determine appropriate rates for small trees.
4. Use the low end of the range of labeled rates, especially in years when spring is late and trees may move very rapidly from Green Tip to Bloom.
5. Avoid applying copper just before predicted frosts or within several days after frosts have occurred.
There is a small possibility that fire blight bacteria can be moved in symptomless nursery trees if the nursery trees are grown in regions where fire blight is prevalent in the surrounding orchards. Streptomycin-resistant strains of the fire blight bacteria have been identified in most fruit-growing regions west of New York and Pennsylvania. To minimize the likelihood that streptomycin-resistant fire blight will be introduced into New York on nursery stock, we recommend that all newly planted apple and pear trees receive two applications of copper in the year they are planted and again the following year. The first application should be made at bud-break and the second should be made about the time the trees would be in bloom. (For newly planted trees, this will be somewhat later than the time that established trees are blooming.) The rate of copper applied to newly planted trees should be reduced to avoid stunting them. The label for COCS specifies 1/4 pound per 100 gallons for sprays after Green Tip, and that rate should be adequate for newly planted trees. Copper applications during Bloom can induce severe fruit russetting, but this is of no concern on newly planted trees because the trees should usually be defruited anyway.
TWILIGHT ORCHARD MEETING, APRIL 29
Mark your calendar now. More information will follow.
_______________________________
David A. Kollas
Telephone: 860-486-1944
dkollas@canr1.cag.uconn.edu