Vermont Apple IPM Newsletter

Lorraine P. Berkett, Gwendolyn Neff, Jon Clements, and Roger Brouillette

May 1, 1997

Stage of Bud Development: Half-Inch Green at the UVM HRC

Disease Management

Apple Scab: We collected about 20 scabby McIntosh leaves from the orchard floor at the UVM Hort. Res. Center (HRC) on Tuesday, April 29 (after Monday's rain) and brought them into the laboratory to check for mature ascospores. Using a technique where we soak the leaves in water and then place them in a "spore tower" to capture any ascospores that are discharged, we collected approximately 100 ascospores - - which is not a high number considering how scabby the leaves are and how many hundreds of pseudothecia were present on the leaves. We also dissected out about 20 pseudothecia and examined their contents under a microscope. Only 4 pseudothecia had any mature ascospores present; none of the pseudothecia observed had discharged any ascospores during the previous day's rain. What does all this mean? It means that we are in the early phase of ascospore maturity (i.e., the Lag Phase) and that the rain on Monday most likely released a small percentage of the total potential ascospore dose in your orchard. This release could be significant if (1) you had a high level of foliar scab last year and thus, a high level of overwintering inoculum and (2) the leaves remained wet long enough for infection to occur. [Note: If your orchard is located in the southern part of Vermont and your trees are farther along in terms of bud development, you should expect ascospore development to be more advanced.]

This year, we are also tracking the maturation of ascospores using the model depicted in the New England Apple Pest Management Guide. This figure and a more detailed explanation of the Lag, Accelerated and Final Phase of ascospore maturation appears on pages 9 and 10 of the 1996-1997 New England Apple Pest Management Guide. As of 11:00 am on May 1, we have accumulated 143 DD from Green Tip at the UVM Hort. Res. Center, which puts us in the middle stage of the Lag Phase. The Lag Phase is the period when ascospores mature slowly with the cooler temperatures that we usually have in spring. [Note: Warm days like we had on Wednesday, April 30, 1997, move maturation along quicker. For example, those same leaves that were put in the spore tower as mentioned above were kept at about 68F over Tuesday night and put in the spore tower again the next morning and over 1000 ascospores were released warm temps. will move ascospore maturity along !]. As the Guide notes, fungicides may not be necessary in this phase in orchards that were well-managed for scab last year and have a potential ascospore dose (PAD) below the action threshold. However, if you are above the threshold, infection periods during this phase are important and your trees should be protected (i.e., your orchard should be protected if the wet weather predicted for today and the weekend turns into an infection period).

Fire Blight (FB): As mentioned in the last issue of the newsletter, it is very difficult to predict when this disease will cause significant damage in Vermont. Last year, FB was more prevalent around the state, which means that the potential carryover inoculum is greater this year. Weather conditions should be monitored closely during bloom to determine the risk of infection. Dr. Paul Steiner at the U. of Md. has identified four requirements for infection to occur:

  1. flowers are open;
  2. at least 198 degree hours greater than 65F have accumulated after first bloom;
  3. at least 0.01"rain or dew that day or at least 0.10" of rain fell the day before; and
  4. an average daily temperature equal to or greater than 60F.

When all of these conditions are met in sequence, infection occurs. First symptoms of blossom infection are predicted to occur when an additional 103 degree days have accumulated (base 55F).

Please note: We received a letter from Jeffrey Santosuosso, Product Manager of Merck, indicating that if you purchased Agri-mycin 17 before March 25, 1997 that you should contact your supplier at once. The product may be below specifications and will be replaced as soon as possible.

Rust Diseases: (The following article was written by Dr. David Rosenberger, Plant Pathologist at Cornell's Hudson Valley Lab, and is reprinted from Scaffolds, April 29, 1997)

The two most common rust diseases on apples are cedar apple rust and quince rust. Cedar apple rust causes yellow or orange lesions on both leaves and fruit of susceptible apple cultivars. Quince rust infects apple fruit, but not leaves. Hawthorne rust is less common and can infect apple leaves but not fruit. Hawthorne rust can cause typical rust lesions on leaves of cultivars such as McIntosh, Empire, and Liberty that are generally considered resistant to cedar rust.

Apple fruit are most susceptible to rust infections during the period from tight cluster through bloom. The tissue that will develop into the apple fruit is exposed just below the blossom and can become infected with rust as soon as the cluster leaves fold back from the flower clusters.

However, in most years rust spores are not abundant until apples reach pink or full bloom.

Wetting and temperature requirements for cedar apple rust infection are noted on page 43 in the 1997 Pest Management Recommendations for Commercial Tree-Fruit Production. [Note: This reference is to the NY State recommendation publication; the requirements are listed below.] Wetting requirements for quince rust infection have not been clearly defined. However, severe quince rust infections usually occur only following extended wetting periods (at least 30 hours, usually >48 hrs) with moderate temperatures (50-75 F) have occurred between tight cluster and late bloom. Under these conditions, more than 50% of fruit on unprotected trees can develop quince rust. More commonly, 5-15% of fruit are affected. Delicious, Golden Delicious, Rome, and Cortland appear to be the most susceptible cultivars for quince rust in the Hudson Valley, but quince rust can occur on nearly all cultivars.

Unlike apple scab, none of the apple rust diseases have a secondary infection cycle. Primary lesions on apple fruit or leaves will never result in secondary spread to other apple leaves. All of the inoculum must come from cedar apple rust galls or quince rust cankers on cedar trees. After this primary inoculum is exhausted (usually about June 15 in the Hudson Valley), no further infections on apple can occur until the following season.

Cultivars vary considerably in their susceptibility to cedar apple rust. Golden Delicious, Rome Beauty, Jonathan, Lodi, Idared, and Mutsu are some of the common older cultivars that are very susceptible. Newer cultivars to add to this list include Fuji, Braeburn, Gala, Cameo, Ginger Gold, Goldrush, and Arlet. Honeycrisp is only moderately susceptible to cedar apple rust. McIntosh and Delicious are considered very resistant.

Approximate Number of Hours of Leaf Wetness Required

for Cedar Apple Rust Infections to Occur on Leaves of Susceptible Cultivars1

 

Average Temperature (F) Degree of Infection
  Light Severe
36 24 -
40 12 24
43 8 10
46 6 7
50 5 6
54 4 5
58 3 5
61 3 4
64 3 4
68-76 2 4
79+ - -

1Information from the 1997 Pest Management Recommendations for Commercial Tree-Fruit Production, A Cornell Cooperative Extension Publication. Based on the data of Aldwinckle, Pearson, and Seem, Cornell University. Assumes that cedar apple rust inoculum (orange, swollen galls) is available at the start of the rain. If inoculum is not already present (dry period prior to the rain), add 4 hr at temps above 50F and 6 hr at temps of 46-50F. Infection is unlikely at temps below 46F if inoculum is not already present.

Arthropod Management

Mite Management: Please refer to the 1997 Update to the 1996-1997 New England Apple Pest Management Guide which was sent out with the last issue of this newsletter. The Update contains information on the use of the miticides Apollo and Savey. Note that to manage resistance to these two materials it is highly recommended that if you used Apollo or Savey last year, you should use an alternate miticide this year, such as Agrimek. In several countries, resistance to Apollo and Savey has appeared after only 5 or 6 consecutive years of 1 application per year.

 Good News !!

Vermont,s Apple Team is cooperating in a project led by Cornell University to establish populations of the predator mite, Typhlodromus pyri, as a biological control for the European red mite in northeast apple orchards. The Cornell University project outlines the current and proposed mite control situation as follows:

Three orchards in Vermont are participating in this collaborative project. Last summer, blossoms and leaves containing T.pyri were shipped from Cornell to Vermont and we distributed them in apple trees within the three orchards. Periodically during the 1996 growing season, leaf samples were sent back to Cornell to see if T.pyri were surviving. Leaf samples taken in the 1996 growing season showed larger populations of T. pyri than anticipated (great news!). Wood samples collected in December to determine if the mites had entered their overwintering sites in the cracks and cervices of the tree bark showed that significant numbers of T. pyri had overwintered at all three Vermont sites. Consequently, the three Vermont sites have been asked to disperse the predators into adjacent trees in the early spring of 1997, in order to figure out the best method for aiding the T. pyri in their natural dispersal (they becoming airborne). Therefore, we winter-pruned wood from the six original trees into which the T. pyri had been introduced and then transferred this wood into six adjacent trees, attaching the smaller prunings with light-degradable rubber bands and propping the larger prunings into the scaffolds of the tree. We will see if this procedure is successful in dispersing the predators ! Our next task in monitoring the establishment of T. pyri comes at bloom. T. pyri feed on many different sources of food (making them a good choice for biological control; their population doesn't crash once they eat up all the European red mites). At bloom, T. pyri move into the flower clusters to feed on small amounts of pollen. Cooperators throughout the northeast will be collecting samples of blossom clusters, which will be sent to Cornell for mite species identification and counting. In addition, we will be sending leaf samples from the T. pyri sites throughout the summer for mite identification and counting in order to monitor the T. pyri population.

Certain insecticides, miticides, and fungicides are known to be toxic to T.pyri, such as Lannate, Vydate, Lorsban, syn. pyrethroids, Carzol and the EBDC fungicides. These materials are being avoided in the sites where we are trying to establish this mite. If we are successful in establishing this predator in Vermont, avoidance of these materials will have to be continued if populations are to be conserved.

 Leafminers: Be on the alert for this insect ! High trap captures have been reported in certain areas of Massachusetts. As of April 29, 1997, we had not caught any moths at the UVM Hort.Res.Center but the warm temperatures could stimulate emergence. Thresholds for red sticky trunk traps are:

Tarnished Plant Bug (TPB): We have begun to capture a few insects on white sticky traps. As a reminder, the following are the thresholds for this insect: