June 27, 2000

GROWING DEGREE DAY ACCUMULATIONS through June 26 at the NWMHRS

Year          2000       1999       1998       1997
GDD42     1314       1434       1465       1007
GDD50       729         860         869         569

GROWTH STAGES:
Pear: Bartlett -- 26mm
Apple: Red Delicious -- 36mm
Sweet Cherry: Napoleon -- 20mm, coloring
Tart Cherry: Montmorency -- 20mm, coloring
Grapes: Chardonnay -- bloom

WEATHER
Rainfall this past week amounted to one inch, while evaporation totaled 1.39". Evaporation exceeds rainfall by 0.39" for the week and by 7.54" for the growing season.

PESTS AND DISEASES
By Gary Thornton, District Fruit IPM Agent

In apples, codling moth trap catches are down again as we approach the end of the first generation. If growers are not trapping they should have their first spray on. Spotted tentiform leafminers adult trap catch is up for the second week in a row. No second generation mines have been reported yet. Green apple aphids are high in some young apple trees. Rose chafers are slowing down, but significant defoliation has occurred in some young trees.

European red mites are building in plums. All plum blocks should be checked and treated if above 5 mites per leaf. Ideally this should be done prior to cherry harvest.

Brown rot and alternaria are a concern in sweet and tart cherries. If you have experienced hail and/or frost injury, keep a tight spray schedule in the pre harvest period. Captan is the only material that will suppress or control alternaria fruit rot. Rovral is no longer labeled after petal fall. Cherry fruit fly emergence has increased this past week. Growers who are not trapping should have a spray on at this time. Alternate middle row sprays work well on this pest.

Grapes are nearing full bloom. Growers should have a spray on to control all major diseases at this time. An important follow-up spray 10-14 days after the pre-bloom spray will be equally important.

MISCELLANEOUS

Plum Pox Virus 

As you can see from the article below, Plum Pox was recently discovered in Canada. It is of utmost importance that all growers who have purchased peach, nectarine, apricot or plum trees from Pennsylvania in the last 5 years check for symptoms of the plum pox virus. Your local MSU Extension office has free plum pox virus pocket guides to aid in this process. The virus in Canada showed up on Fantasia Nectarines. Any growers with Pennsylvania sourced Fantasia Nectarines should let us know at the NWMHRS ASAP.

Plum Pox Virus Confirmed in Canada

Ottawa, June 23, 2000. The Canadian Food Inspection Agency (CFIA) today announced the discovery of plum pox virus (PPV) in an orchard outside Niagara-on-the-Lake, Ontario. Sample testing by the CFIA Centre for Plant Health in Sidney, B.C. confirmed the infection. This is the first discovery of plum pox virus in Canada.

Plum pox virus, also known as sharka, is a serious disease infecting stone fruit species of the genus Prunus including peaches, nectarines, plums, apricots and ornamental varieties. PPV does not kill trees but it renders the fruit tasteless and low in sugar, making it unsuitable for eating and processing. There is no treatment for infected trees. They must be destroyed to prevent the spread of the disease. PPV is spread over short distances by aphids and over larger distances through grafting of infected budwood or nursery stock. PPV does not affect humans.

The strain of the virus found in Canada has been identified as Strain D. The same strain of PPV was found in Prunus trees in Pennsylvania in October 1999. This was the first time PPV was found in North America. The CFIA suspended the importation of Prunus plant material from the U.S. in November 1999. The nectarine trees outside of Niagara-on-the-Lake confirmed to have PPV were imported from Pennsylvania.

All Prunus plant material (including nursery trees, scionwood and rootstocks) imported from Pennsylvania in the last three years have been placed under a prohibition of movement, and surveying of this material for the presence of PPV began this spring. The CFIA has initiated a PPV Emergency Program to intensify surveys to assess and determine the extent of the disease and take actions to control its spread.

The CFIA is consulting with the National Plum Pox Virus Task Force, a group of scientists, provincial advisors and industry representatives on activities to prevent and control the spread of this disease. The CFIA will continue to provide updates on the progress of the survey and eradication activities.

Bacterial Canker
By Jim Nugent

Bacterial canker (BC) can invade cherries through wood, leaves and/or fruit. It is favored by extended periods cool, wet weather. These conditions were common during several extended periods since bloom. There is some fruit injury showing up this year, as well as some leaf injury.

The extent of leaf injury can be greatly affected by the spray program. In particular, sprays that have some tendency toward phytotoxicity can lead to significant invasion of the causal organisms into the leaves. Often it appears the spray-related problems are due to a combination of materials.

What's unusually prevalent in 2000, however, is invasion of BC into spurs on sweet cherries. The spur invasion was predisposed by a late frost event (or at least very cold temperatures) following bloom that was then followed by a lot of cool, wet weather. This combination of events is very unusual. The result has been very significant spur loss in some blocks, particularly in low areas.

There is no cure for the spur loss problem. Try to keep trees as healthy otherwise as possible. Make sure pH is above 6.2. Consider copper during the dormant season (late fall and/or early spring). Do not apply copper now! This could lead to much more serious problems. 

Fire Blight Rundown
(Dave Rosenberger, Plant Pathology, Highland)
In Scaffolds Fruit Journal, June 26, 2000

In the last three weeks, severe fire blight has been reported in a number of apple orchards throughout the Hudson Valley. The blight outbreak this year has several unusual characteristics. Pear blocks are mostly unaffected because pears were at petal fall when the blight infections occurred. In apples, most of the fire blight in the Hudson Valley has developed in Gala blocks where trees are three to five years old. In several cases, Gala blocks are so severely affected that several hundred trees have already been removed and many more will be lost to rootstock blight. 

At this point, the incidence of blight appears manageable in adjoining blocks of other cultivars including Cortland, Rome, Spartan, Macoun, and Delicious. The distribution of blight strikes in cultivars adjacent to affected Gala blocks suggests that Gala acted as a source of inoculum for other cultivars. The number of blight strikes in those blocks is greatest immediately adjacent to or downwind from the Gala trees. In several of the Gala blocks, we discovered evidence of over-wintering cankers from last year or point-sources for this year's infections, even though the growers involved were not aware that fire blight had been present in the orchard last year. 

The good news is that the fire blight epidemic in the Hudson Valley is still limited to relatively few orchards. The devastating hailstorms that occurred beginning May 19 did not generate the disaster that would have developed if blight inoculum had been present in all orchards. The hailstorms did contribute to spread of blight to adjacent cultivars in some of the affected orchards, but there are still many orchards, including Gala blocks, that do not have any fire blight at this time.

What is the best way to deal with fire blight in young orchards where blight was not completely controlled during bloom? No single answer can be applied to all situations, and there is considerable room for debate on many details relating to blight management during summer. Dr. Paul Steiner at the University of Maryland has posted some excellent articles about fire blight on the World Wide Web at www.caf.wvu.edu/kearneysville/wvufarm10.html.  However, none of the articles answers all of the questions that arise after blight appears in an orchard. Answers for many questions are lacking because the appropriate research has not been done or because research results have been inconsistent.

Growers dealing with fire blight must make daily management decisions even when scientists do not have enough data to provide research-based recommendations. Therefore, I have provided below my "best guesses" for some of the questions raised by growers. Much of this information may come too late to be useful for this season, but it may help to stimulate discussion and awareness concerning the best approaches for managing shoot blight in the future.

1. Q: Should I try to prune out fire blight when it appears in young trees? 

A: Absolutely, unless blight is so severe that the orchard is beyond hope. Strikes should be pruned out as soon as possible after they appear. Failure to do so increases the likelihood that blight will continue to spread both to adjacent trees and into the rootstocks of affected trees. Pruning out infections in mature trees may not be practical, but mature trees with a full crop will set terminal shoot buds earlier than young trees. When trees set terminal buds, blight stops spreading both between trees and within the affected trees. 

In order to remove strikes before cankers extend too far into the tree, trees must be examined at least two or three times weekly until the epidemic begins to slow. In sections where trees are severely affected, it may be more cost-effective to immediately remove entire trees, especially if trees are a susceptible cultivar like Gala. Pulling out badly affected trees will allow blight removal crews to focus their efforts on trees that can be salvaged.

Blight removal crews should be trained to recognize the early symptoms of blight on terminal shoots. On terminals just beginning to show symptoms, the first or second fully expanded leaf will droop and closer examination will show blackening along the midvein at the base of the leaf blade. The entire shoot tip may appear to be slightly yellowed. Remove such shoots by cutting back into two-year-old wood at least 8-12 inches 
below the last visible symptoms. If a spur or shoot on the central leader shows signs of blight, immediately remove the central leader down to 8-12 inches below the last visible symptom. Immediate and aggressive removals reduce the need for repeated pruning in the same tree and may result in fewer trees lost to root stock blight.

2. Q: Is it necessary to disinfect pruning tools between cuts?

A: Dr. Paul Steiner has shown that disinfecting pruning tools is a waste of time because minute cankers often form on the ends of cuts even when pruners are disinfected. Instead of wasting time disinfecting pruning tools, Paul recommends making all cuts into at least 2-year-old wood where bacteria will be less able to multiply. Also, leave "ugly stubs" by cutting branches between nodes and at least several inches away from the central leader. Small cankers that form on these stubs can then be removed during winter pruning whereas a canker that forms at a flush cut on the central leader will be missed during winter pruning. 

An extension specialist in California reported that he failed to transmit fire blight with pruning tools when he purposely made cuts through active cankers in dry weather. However, he succeeded in transmitting blight on pruning tools when pruning was done in wet weather. Blight removal operations should usually be suspended in wet weather, but that is not always possible. (See question #4 below). As a precaution, perhaps pruning tools should still be disinfected if blight removal must be done in wet weather.

3. Q: Should prunings be removed from the orchard?

A: I haven't found any recent recommendations addressing this question (although I admit my search was not exhaustive). My personal recommendation is to place prunings in the row middles and allow them to thoroughly dry before running a mower over them. "Thoroughly dry" means that the bark no longer slips on the branches that have been removed, and the out bark and cambium have turned brown. With today's tightly spaced orchards, I am concerned that carrying prunings out of the orchard may spread more blight than occurs when prunings are left to dry in the row middles. 

4. Q: What about pruning out blight in damp or rainy weather?

A: In the ideal world, blight removal would only be done in dry weather. However, when a week of rain is predicted just as the first symptoms of blight appear, one must weigh the risks of spreading blight by pruning in wet weather versus the risks of giving the epidemic a full week, or even a two- or three-day head start. With highly susceptible cultivars like Gala, I would opt to remove blight as quickly as possible, even if that meant that some removal would be done in less than ideal weather. 

5. Q: Can I do hand thinning or bud pinching while blight is active in the orchard?

A: Avoid these activities until after terminal bud set. Delaying hand thinning may result in some loss of fruit size, but risks of spreading blight outweigh the benefits of early hand thinning. One local grower demonstrated that pinching buds as part of tree training for the vertical axe system is a great way to spread blight. Even though we no longer recommend disinfecting pruning tools between cuts, one can still spread blight on one's fingers while pinching buds (and presumably while hand-thinning). Pinching is done to succulent shoot tips that are highly susceptible to blight, whereas cuts made to remove blight are made in wood that is at least two years old. 

6. Q: What can be done to stop the spread of blight to new terminal shoots?

A: No good answers here. Anything that helps to shut down tree growth will help to limit the spread of blight since the epidemic stops when terminal buds are set. Lucky growers never get blight, but if they do, they only get it in drought years when trees stop growing in mid-June. (This is not a lucky year!) Obviously, blocks with blight should not be trickle-irrigated until well after terminal bud set. Allowing weed regrowth beneath trees may increase competition for water and nutrients, thereby helping to slow tree growth.

The new plant growth regulator called "Apogee" may prove useful for arresting blight epidemics. This product has a Federal label, but it is not yet registered in New York State. Apogee causes trees to set terminal buds beginning about two weeks after it is applied. To control vegetative growth in overly vigorous blocks, Apogee application will be recommended at late bloom or at petal fall. In young orchards, however, early cessation of terminal growth is undesirable except when blight is present. If Apogee is applied after the first symptoms of blight appear in an orchard, two applications will probably be needed to rein in the growth process and blight will continue to spread for at least two weeks after the first Apogee application. In highly susceptible cultivars, blight may reach the rootstocks in many trees before Apogee can shut down terminal growth and make the tree more resistant to blight. In dry years, untreated trees may stop growing on their own about the same time that Apogee takes effect. The earlier cessation of shoot growth triggered by Apogee will help control blight in wet years, but the combined cost for the two Apogee treatments may exceed $150 per acre. Cost-effectiveness of Apogee for fire blight remains to be determined. 

Until the mid 1980's, fire blight experts recommended reducing action thresholds for aphids and leafhoppers in orchards with blight because of concerns that these insects might spread blight during summer. Research in the Mid-Atlantic States has shown that aphids and white apple leafhoppers cannot vector blight. The role of potato leafhoppers (PLH) is less clear. A spray of Provado or carbaryl to control PLH may be helpful. However, complete control of PLH is impossible in a season when thunderstorms regularly drop new immigrant PLH adults into orchards.

Streptomycin sprays should NOT be applied during summer because summer applications will result in rapid development of streptomycin-resistant strains of the blight pathogen. The only exception is that streptomycin should be applied immediately after any hailstorm if there is active blight in the orchard (i.e., orchards where blight was present this year and terminal shoots are still growing).

Copper sprays applied in summer theoretically should inactivate blight bacteria on plant surfaces and thereby help to reduce inoculum and slow the epidemic. However, attempts to document the benefits of summer copper sprays have provided inconsistent results. Proponents of using copper during summer admit that benefits of copper will be limited because copper is not systemic and therefore will not affect bacteria inside plant tissue. Furthermore, actively growing terminal shoots "outgrow" the copper residue, thereby leaving the blight-susceptible shoot tips unprotected within several days after an application. Copper applied in summer is also phytotoxic to fruit, with injury appearing as necrotic black spots at fruit lenticels. Thus, copper sprays are not acceptable where the crop is destined for fresh market. In young orchards, salvaging the crop may be less important than salvaging the trees. This is especially true this year when many orchards in the Hudson Valley already have severe hail damage. 

The bottom line: If I was managing a young Gala block with fire blight, I would be applying a low rate (about 4 oz/100 gallons dilute spray) of a fixed copper on a 7-10 day schedule until terminal buds are set. Copper sprays should be applied under good drying conditions. The alkaline nature of copper sprays means that they probably cannot be combined with other pesticides that are subject to alkaline hydrolysis.

7. Q: What determines how many trees will develop rootstock blight?

A: No one knows. Rootstock blight develops when bacteria move from strikes in the top of the trees down through the trunk and cause cankers in the rootstock. Because M.9 and M.26 rootstocks are highly susceptible to blight, rootstocks that become infected usually die. In the Hudson Valley, Gala orchards that showed the first symptoms of fire blight in early June now have many trees with bacterial ooze coming out of the rootstocks. Some of the trees with rootstock blight still have a reasonably intact canopy, but they will not survive. Some will die within several weeks, some will die later this fall, and some will survive until next spring when they will wilt and die soon after bud break. Incidence of root stock blight can range from less than 5% of trees to more than 80% in a severely blight block. Rootstock blight is most common in orchards less than 6 years old, but other factors that make trees susceptible to rootstock blight have not been determined. 
 
 

2000 Tree Fruit Crop Estimates
		Actual			Estimate
Tart Cherries (million lbs.)	1997	1998	1999	3 yr. avg.	Processor est.2000 *	USDA est.2000
NW MI	140.0	186.0	108.5	144.8	88.0	90.0
West Central MI	70.0	59.0	47.5	58.8	51.3	57.0
SW MI	15.0	18.0	29.0	20.7	16.2	18.0
Total MI	225.0	263.0	185.0	224.3	155.5	165.2
Total U.S.	292.9	348.1	256.1	298.8	228.5	245.4
Sweet Cherries (million lbs.)
Michigan		70.0	56.0		37.8	44.0
U.S.		423.0	449.3		NA	448.8
Apples (million bu.)
Michigan	25.0	23.1	29.8	26.0	22.5	NA
Washington	119.0	152.4**	121.4	131.0	153.0	NA
U.S.	247.3	271.1	255.7	258.1	272.0	NA
* Michigan Food Packers Assoc., June 22, 2000
** Current record for WA

Marketing Order Scenario Based on USDA Crop 
Forecast for 2000 Supplies
 

U.S. Crop (USDA Est.) 245 M lb
Carry in                           +88
Total Available               333
  Optimum Supply Formula 3 yr. avg. sales               275
Target carry-out                +0
Optimum supply             275
  Surplus                              58
Adjustment for diversion 
Qualifying export             +35
Adjusted surplus               93


Crop restricted 
districts (USDA Est.)                      195

Initial restriction                              47.7%

Market growth                    28 M = 14.4%
factor release            
(Required release from 
restriction in January)

Effective restriction                     65 M

of this 65 M restriction, it is thought that the industry might export about 35M and put 5 M into qualifying new products/new markets, leaving about 25 M, or 13%, for non-harvest and and/or reserve pool.

Please send any comments or suggestions regarding this site to: 
Bill Klein, kleinw@pilot.msu.edu

Last Revised:6-29-00