Fruit Pest News
May 7, 2001
A weekly, online newsletter whose goal is to update Extension agents and growers of commercial tree fruit and small fruit crops on diseases and insects in Tennessee.
Text appearing in blue or red can be clicked to link to other web sites. Be aware that much of the linked information is produced in other states and may not be applicable to Tennessee.
In This Issue:
1. Current Crop Situation
2. Stone Fruit: Plum Pox Survey Plans
3. Fire Blight: Secondary Blooms
4. Fire Blight: Shoot Blight and Canker Blight
5. Fire Blight: When to Stop MaryBlyt Data Entry
6. Tree Fruit: Insect Update
1. CURRENT CROP SITUATION
Dry weather continues to dominate the crop production picture. Scattered thunderstorms reportedly occurred on three consecutive days in East Tennessee last week. Thunderstorm activity is forecasted for several days this week. Strawberry growers have benefited from the dry weather during harvest. Apple growers are seeing the fire blight that resulted from the warm weather during bloom. Cedar-apple rust spores are still available, and protection is needed during rainy spells. (SB)
2. PLUM POX SURVEY PLANS
The second year of the survey of stone fruit trees for the plum pox virus (PPV) is about to get underway. Anni Self, of the Tennessee Department of Agriculture, will soon be leading a survey team to collect 600 samples. The preferred time for PPV sampling is in the spring and fall, because detection of the virus can be impeded by high temperatures. The year's survey has been delayed somewhat by the release of funds from USDA-APHIS-PPQ.
Everyone associated with the stone fruit industry should be grateful for this survey, as it protects us by determining the whereabouts of this disease. Please help us by cooperating with the survey team any way you can. Nurseries and orchards will be included. Thus far, the only North American plum pox cases have been in Pennsylvania and Ontario.
Currently, there is a voluntary budwood testing program in place. Anyone planning on collecting budwood for propagation purposes is asked to have each mother tree tested for the plum pox virus. Tennesseans, please call Anni Self at 615-837-5313 and let her know that you plan to collect budwood. She can sample the trees for you and have them tested for PPV at no charge to you (It is a good idea to test for other viruses such as prunus necrotic ringspot and tomato ringspot while you are at it, but you will have to pay for those tests). (SB)
3. FIRE BLIGHT: SECONDARY BLOOMS
Most apple varieties have finished blooming, with the exception of secondary (tag-on) blooms. I received a report of a block of Romes in East Tennessee still in bloom. The MARYBLYT program ceases to predict fire blight infection periods and the need for streptomycin sprays after all blooms are gone. We do not recommend the use of streptomycin after bloom because of its ineffectiveness against shoot blight infections, and because such use increases the risk of resistance developing. Beware, however, of tag-on blooms. Streptomycin sprays may be needed as long as they are present. In such warm weather, tag-on blooms can easily become infected and negate all your hard work in preventing infections during the bloom period! If only a few secondary blooms are present, it would be worth your while to remove them by hand, because they potentially represent the beginning of a fire blight epidemic!
4. FIRE BLIGHT: SHOOT BLIGHT AND CANKER BLIGHT
It is shaping up to be a significant fire blight year due to the warm weather we had during bloom. One grower, whose orchard has a history of fire blight problems, reports improved control using MaryBlyt to guide him on his streptomycin applications. Regardless of the amount of blossom blight you are seeing, shoot blight remains a threat. Have you ever experienced severe shoot blight in a year when you saw little or no blossom blight? Probably so. That's because overwintering cankers can produce the bacteria that infect the shoots (Hopefully, you removed overwintering cankers before growth began in the spring).
Shoot blight is very important because it can destroy a sizable portion of the fruiting wood and because it often creates a canker that serves to overwinter the bacteria for the next season.
Causation. Leaf surfaces are colonized by the bacteria arriving from infected blossoms, overwintering cankers, or canker- blighted shoots*. The long-held belief that sucking insects are involved in introducing the surface bacteria into the leaves is now called into question. Green apple aphids and white apple leafhoppers are no longer thought to be involved. There is mounting evidence that gusty winds are involved by causing small injuries to tender shoot tips, allowing the surface bacteria to enter.
Management. Recommendations for management of shoot blight have changed in light of the recent evidence that insects may not play a large role, and because of the recent registration of an effective control product. The plant growth regulator Apogee is an important management tool for shoot blight in that it slows shoot growth, thus decreasing its susceptibility. However, the use of Apogee will not eliminate shoot blight, and it is still important to continue basic efforts to reduce fire blight inoculum. If it is practical, you should prune out shoot blight symptoms as soon as they appear. If it is not possible to complete this task within a couple of days, it is probably best to let nature take its course.
The knowledge that wind gusts can cause shoot blight is not particularly useful, since it is not practical to spray every time the wind gusts more than 8 to 10 mph during rapid shoot growth, even if there were an effective bactericide. Streptomycin has proven to be ineffective in preventing shoot tip infections and copper products have the potential for phytotoxicity. The only other products labeled for use on apple that may provide control of shoot blight are Aliette and Messenger, but I am not aware of any data that supports their recommendation for this purpose.
*A type of fire blight infection that can be confused
with shoot blight is canker blight, which is the blighting of shoots located near
overwintering cankers. Canker blight can be severe in light blossom blight years,
because it is the result of fire blight infections that took place the previous year.
Shoots infected the previous year can lead to overwintering cankers that resume activity
in the current year. The bacteria invade nearby shoots internally, causing them to
wilt and die. Such systemically infected shoots can be distinguished from shoot blight
by the early yellow to orange discoloration of the tip bud, stem, and leaf midribs.
Shoot blight is characterized by a wilting of green shoot tips.
5. WHEN TO STOP MARYBLYT DATA ENTRY
If you are operating the MaryBlyt computer program for fire blight management, there is a good chance that your program has, by now, indicated the appearance of the three common symptoms of fire blight: blossom blight, canker blight, and shoot blight. I found that the program was quite accurate in predicting the date of appearance of the first two types in the orchard here at the office. Shoot blight prediction is less exact and more prone to error; refer to article above.
The two main purposes of MaryBlyt, to assist in the timing of streptomycin applications during bloom and in the timing of removal of fire blight symptoms, have largely been satisfied at this time. However, trauma blight, caused by hail or high winds, remains a possibility at any time. MaryBlyt will alert you as to when to expect the first symptoms to appear after a trauma event so that you can remove them if you wish. For this purpose, you should continue to enter daily temperature data into your program. Alternatively, you can calculate the expected time of symptom development yourself, using Table 2 on page 52 of the MaryBlyt manual. Once injury occurs, symptoms can be expected following an interval of 103 degree days > 55° F. (SB)
6. TREE FRUIT: INSECT UPDATE
For OFM, for moderate to high density OFM
orchards (caught more than 10 moths/trap/week), two insecticide applications
should be made 14 days apart, the first at 55DD after biofix. We had 501.8 DD on
May 4 and the orchard was sprayed that day. As of May 6 there were 556.8 DD.
The
first spray for codling moth was our petal fall spray on April 20 when we had just
exceeded the 250DD threshold with an actual 253DD. A second spray is needed 14 days
later which corresponds nicely to our May 4 spray.
(FH)
Obion Co. (traps
put out April 9)
| Date | OFM | CM | RBLR |
| 4-13 | 0 | 21 | 28 |
| 4-20 | 6 | 0 | 3 |
| 4-27 | 0 | 0 | 0 |
| 5-4 | 2 | 0 | 0 |
| Davidson Co. | ||||
| Date | OFM | CM | RBLR | OBLR |
| 3-26 | -- | -- | 8 | -- |
| 3-28 | -- | -- | 0 | 0 |
| 3-30 | -- | -- | 8 | 0 |
| 4-2 | -- | -- | 10 | 0 |
| 4-4 | 0 | 12 | 13 | 0 |
| 4-6 | 0 | 1 | 13 | 0 |
| 4-12 | 23 | 5 | 53 | 0 |
| 4-16 | 11 | 1 | 12 | 0 |
| 4-19 | 0 | 0 | 0 | 0 |
| 4-20 | 0 | 0 | 0 | 0 |
| 4-23 | 12 | 2 | 0 | 0 |
| 4-25 | 5 | 0 | 0 | 0 |
| 4-26 | 0 | 0 | 0 | 0 |
| 4-27 | 2 | 0 | 0 | 0 |
| 4-30 | 6 | 0 | 0 | 1 |
| 5-3 | 35 | 0 | 1 | 17 |
| 5-4 | 3 | 0 | 0 | 8 |
| 5-7 | 12 | 0 | 0 | 10 |
| Putnam Co. | |||
| Date | OFM | CM | RBLR |
| 4-6 | 4 | 0 | 15 |
| 4-9 | 15 | 1 | 9 |
| 4-11 | 12 | 0 | 67 |
| 4-13 | 3 | 0 | 37 |
| 4-16 | 0 | 0 | 14 |
| 4-18 | 1 | 0 | 1 |
| 4-20 | 1 | 0 | 8 |
| 4-23 | 0 | 0 | 11 |
| 4-24 | 2 | 0 | 1 |
| 4-27 | 0 | 0 | 0 |
| 4-30 | 0 | 0 | 0 |
| 5-2 | 0 | 0 | 0 |
| 5-4 | 1 | 1 | 0 |
| Bradley Co. | |||
| Date | OFM | CM | RBLR |
| 3-15 | 2 | 0 | 0 |
| 3-23 | 1 | 0 | 5 |
| 4-5 | 0 | 0 | 2 |
| 4-7 | 0 | 0 | 1 |
| 4-9 | 1 | 0 | 0 |
| 4-12 | 3 | 0 | 0 |
| 4-13 | 2 | 0 | 0 |
| 4-15 | 3 | 0 | 7 |
| 4-22 | 0 | 2 | 2 |
| 4-24 | 0 | 5 | 0 |
| 4-25 | 0 | 4 | 0 |
| 4-28 | 2 | 8 | 0 |
| 4-29 | 0 | 10 | 1 |
| 5-1 | 0 | 5 | 0 |
| 5-4 | 3 | 4 | 0 |
| 5-6 | 0 | 6 | 0 |
The Fruit Pest News URL is: http://web.utk.edu/~extepp/fpn/fpn.htm
Contacts:
Steve Bost, Professor and Extension Plant Pathologist
Frank Hale, Associate Professor and Extension Entomologist
Both authors available at:
615-832-6802
fax 615-781-2568
Plant and Pest Diagnostic Center
5201 Marchant Drive
Nashville, TN 37211