Publication 360, Fruit Production
Recommendations: Apple Calendar - Notes on Apple Diseases
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Excerpt from Publication 360, Fruit Production Recommendations 2010-11,
To order this publication
Table
of Contents
- Notes on Apple Diseases
- Table 4-12. Relationship of Temperature and Moisture to Apple Scab Infection PDF
- Table 4-13. Characteristics of Apple Scab Fungicides
- Table 4-14. Activity of Fungicides on Apple Diseases
- Other Information on Apples
- Chapter 4: Apple Calendar - PDF 357 kb
- Related Links
Notes on Apple Diseases
Apple scab
Primary infection periods
Researchers at Cornell University have developed a model that relates
temperature to spore maturation. The model is expressed in degree-days
Celsius (DDC) and allows you to predict the percentage of the season's
ascospores that have matured. For further information, see Figure
4-2. Cumulative Percentage of Ascospores Matured at Various Degree-day
Accumulations, on this page. Daily accumulated degree-days are calculated
as follows:
DDC = (Daily max. °C + Daily min. °C) /2 - 0°C
For example, on a day with a high of 10°C and a low of 2°C,
the accumulated degree-days are six DDC. If either of the daily
temperatures is below 0°C, i.e. a negative number, then use
a value of zero for that temperature. Begin degree-day accumulations
at bud break, which is defined as the day on which at least 50%
of the fruit buds on McIntosh are between silver tip and green tip.
Use the degree-day accumulations to estimate the percentage of mature
ascospores from the central curve in Figure 4-2. The upper and lower
curves are the upper and lower 90% confidence limits around the
estimates on the graph. The estimate should fall within this range
90% of the time.
Figure 4-2. Cumulative Percentage of Ascospores
Matured at Various Degree-day Accumulations
Two points of particular interest on this graph are:
- At 125 DDC, there is a rapid maturation of ascospores that indicate higher risk of infections.
- At 418 DDC, over 95% of the ascospore supply should be depleted if sufficient rain has occurred. This marks the end of the primary infection season. With this model, growers can obtain a site-specific forecast of ascospore maturity and discharge for their own orchard(s).
Ascospores are released when there is rain to wet the pseudothecia
in the dead leaves. Most of the available mature spores are discharged
within two hours after the start of rain. Ascospore release is strongly
light dependent and only a small percentage of available ascospores
are released at night from 7 p.m. to 8 a.m. Eastern Daylight Savings
Time [EDST].
In low inoculum orchards ascospores are not released to
any significant extent during night-time hours. In an orchard where
the ascospore inoculum is high, although the percentage of ascospores
released is small at night, the total number of ascospores released
is large and can result in a significant primary scab infection.
If scab was present in the orchard the previous season, consider
the orchard to have high inoculum.
In a low-inoculum orchard, calculate the length of the wetting period using the following method:
- When rain begins during the day, between 8:00 a.m. and 7:00 p.m. EDST, count the hours of leaf wetness from when the first hour rain was recorded until the leaves are dry.
- When rain begins at night, between 7:00 p.m. and 8:00 a.m. EDST, count the hours of leaf wetness from 8:00 in the morning until the leaves are dry.
For a high-inoculum orchard, calculate the length
of the wetting period from the start of the rain until the leaves
are dry, regardless of the time of day. In high-inoculum orchards
if a rain event starts during the day, continue counting the hours
of leaf wetness even after nightfall.
Rainfall is needed for the release of ascospores and the leaves
and fruit must be wet for infection to occur. The length of the
wetting period required for infection varies with temperature. The
relationship between hours of wetting and temperature is outlined
in Table 4-12. Relationship of Temperature and Moisture to Apple
Scab Infection, below.
|
Average temp. (°C)
|
Minimum number of hours of leaf wetness
required
Primary season (ascospore infection) |
Minimum number of hours of leaf wetness
required
Secondary season (conidia infection) |
|---|---|---|
|
1
|
40
|
37
|
|
2
|
34
|
33
|
|
4
|
27
|
26
|
|
5
|
21
|
23
|
|
6
|
18
|
20
|
|
7
|
15
|
17
|
|
8
|
13
|
15
|
|
9
|
12
|
13
|
|
10
|
11
|
12
|
|
11
|
9
|
10
|
|
12
|
8
|
9
|
|
13
|
8
|
9
|
|
14
|
7
|
9
|
|
15
|
7
|
9
|
|
16
|
6
|
9
|
|
17
|
6
|
8
|
|
18
|
6
|
8
|
|
19
|
6
|
8
|
|
20
|
6
|
7
|
|
21
|
6
|
7
|
|
22
|
6
|
7
|
|
23
|
6
|
8
|
|
24
|
6
|
9
|
|
25
|
8
|
11
|
|
26
|
11
|
14
|
Adapted from: Stensvand, Gadoury, Amundsen, Semb and Seem, 1997, Phytopathology 87: 1046-1053.
Once the length of the wetting period is determined, calculate
the average temperature during this interval and check Table 4-12.
Relationship of Temperature and Moisture to Apple Scab Infection,
to see if leaves were wet long enough for an infection to occur.
Once a primary infection occurs and primary lesions are present,
then secondary infections can occur. Initially lesions can be very
difficult to see. As a result, scouting and maintaining adequate
fungicide coverage is very important.
Secondary infection periods
Secondary scab infections occur when splashing rain spreads conidia,
developed from primary scab lesions, on leaves. Secondary infections
only occur if moisture from rain is present long enough at a given
temperature. Since secondary infections can occur day or night,
calculate from the beginning of the wetting period, regardless of
time of day.
Effect of intermittent rain and dew on infection periods
Periods of dew or high humidity (over 90%) also contribute to a wetting period, but are significant only if preceded by rain. Add together wet periods caused by intermittent rain to determine the length of an infection period, unless wet periods are separated by 10 hours or more of dry, sunny weather.
Fruit infection
As the fruit matures it takes a longer wetting period for infection by apple scab to occur. Refer to OMAFRA Publication 310, Integrated Pest Management for Apples for more information.
Reducing apple scab inoculum
While growers have little control over weather conditions, they can reduce inoculum and ensure adequate fungicide protection. The following methods will reduce inoculum in orchards.
The application of urea (45 kg of agricultural urea per 1,000 L of water/ha) to the orchard floor after about 95% leaf drop (November) or in the spring (April) before bud break reduces the number of ascospores. The urea works in two ways. It directly inhibits the development of ascospores, and it stimulates the growth of naturally occurring organisms that are antagonistic against the fungus that causes apple scab, Venturia inaequalis. In most years the spring treatment is more effective and results in fewer leaf and fruit infections. When snow cover remains until bud break there is not a lot of time for the urea to work, and the treatment is less effective.
Shredding overwintering leaves using a flail mower can also reduce primary inoculum. Rake or blow leaves from under trees and shred them using a flail mower. Shredded leaves decay more quickly.
Management of apple scab with fungicides
Early season management
Management programs for primary apple scab infection are based on the timing and type of fungicide used. There are two general approaches. Apply fungicides as a protectant program before infection occurs, or apply fungicides after infection but before symptoms develop.
Fungicides are applied to expanding foliage before infection periods
occur. Apply fungicides when conditions for infection are imminent.
Begin fungicide programs between silver tip and green tip. From
first cover until early August, apply fungicides at 10-14 day intervals
depending on rainfall frequency. New growth and expanding fruit
must be protected and residues washed off by rain must be replaced.
The exception to this is when using DMI fungicides (Nova and Nustar).
Use a 5-7 day spray schedule regardless of rainfall. Some fungicides
are not readily washed off and the residues are redistributed by
rainwater. See Table 4-13. Characteristics of
Apple Scab Fungicides.
There are several different fungicide groups available for use in
apples. See Table 2-11. Fungicide/Bactericide Groups Based on Sites
of Action. For more information on these fungicides refer to OMAFRA
Publication 310, Integrated Pest Management in Apples. For information
on resistance management strategies with these products refer to
Pest Resistance
to Insecticides, Fungicides and Miticides, and Resistance
management recommendations by fungicide group and disease for Ontario
fruit crops.
Control of apple scab under adverse weather conditions
Several consecutive days of continually wet conditions and mild temperatures are common in the spring. These conditions encourage apple scab infection. It is often difficult to maintain sufficient fungicide on the trees to provide protection for the rapidly expanding foliage and fruit under such conditions.
If there is concern about adequate protection, it is better to apply a protectant-type fungicide during a break in the rain than to not spray at all. Be sure to use a fungicide with good retention properties. For example, products with mancozeb such as Dithane, Manzate and Penncozeb have good retention activity.
After-infection sprays may be required when protectant fungicides become too diluted to prevent the apple scab fungus from establishing itself on the foliage or fruit.
If an infection exists, scab lesions become visible on the foliage and fruit in about nine days at 20°C or in about 14-21 days if the average temperatures are 12°C or lower. Once this occurs, avoid using fungicides with high potential for resistance (DMI's, strobilurins) and focus on protecting uninfected tissue from further infections.
Strong winds can hamper spray operations by causing pesticides to drift away from the intended target. Spray at night or early morning to help reduce spray drift. You can also lower the spray trajectory into the wind to minimize drift, but take extra care to ensure the treetops get good spray coverage.
Secondary scab management
The end of primary season can be determined through use of the degree-day model. At 418 DDC, over 95% of the ascospores are mature, and after a significant rain all of the ascospores have been discharged for the season. After this point, wait two weeks and then check the trees for scab. It can take up to 14 days for lesions to appear after a scab infection period. Be sure to check the top of standard trees or anywhere else spray coverage may not have been adequate, such as a thick tree canopy.
If primary scab was controlled, the rates of fungicides may be reduced and the interval between sprays may be lengthened for the remainder of the growing season. Some growers have successfully eliminated all use of fungicides during the summer months where primary scab was controlled. If scab lesions are found, maintain a fungicide program for the remainder of the season.
Choosing a fungicide
Use the information in the following tables to assist with choosing the best product for the most likely diseases. Consider the history of the disease, resistance management strategies, as well as the activity of each product to diseases, and the weather. See Table 4-13. Characteristics of Apple Scab Fungicides, and Table 4-14. Activity of Fungicides in Apple Diseases.
*Calculated from the start of the infection period
Data is adapted from New York (Geneva) from work done by Szkolnik
et al, using conidia. These values are adapted from greenhouse tests
on Golden Delicious. The after-infection activity of these fungicides
may not be adequate to control primary scab in commercial orchards.
Do not rely solely on after-infection activity for the control of
apple scab.
These results are formulation specific. Newer products and formulations
of some products have not been evaluated (e.g. Dithane DG, Polyram
DF, Manzate 200DF and Pristine WG). Contact the manufacturer for
more information.
1 Maximum after-infection activity is calculated from the start
of the infection period
2 Data on Nova is adapted from 1993 Pest Management Recommendations
for Commercial Tree Fruit Production, Cornell Cooperative Extension,
New York.
3 After-infection activity of DMI (Nova and Nustar) and
QoI (Flint and Sovran) fungicides may be reduced in orchards where
scab populations have shifted toward resistance to DMI fungicides.
Blank cell means information is unavailable.
KEY: E = excellent; VG = very good; G = good; F = fair; P = poor;
N = none. Values followed by '*' are based on field observations.
Fungicide resistance
For more information see, Pest Resistance to Insecticides, Fungicides, Miticides, and Resistance management strategies.
Ratings in shaded cells indicate the disease is listed on the product
label for control or suppression. Use fungicides only for diseases
listed on the product label for the crop and for the disease. Additional
information is provided in this table to assist the grower in choosing
the best fungicide for control of diseases listed on the product
label.
P = Phytotoxic, causes russeting.
R = Resistance has been reported in some Ontario orchards. If you
are uncertain whether there is a problem with resistance in your
orchard, have it tested.
0 = No control.
NA = Not applicable because the fungicide is not applied at the
timing for this pest.
+ = Poor to fair control; ++ = Good control, some limitations; +++
= Excellent control, few if any limitations.
Source: various extension publications and Plant Disease Management
Reports (APS).
Learn more:
Related Links
- Apples in Ontario
- Integrated Pest Management (IPM)
- Pesticide Application and Safety
- Pesticide Companies and Agents
- Search the PMRA website
- Ontario Minor Use Program
- OMAFRA Publication 505: Ontario Weeds
- OMAFRA Publication 75: Guide To Weed Control: Tree Fruit
For more information:
Toll Free: 1-877-424-1300
Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca
| Author: | OMAFRA Staff |
|---|---|
| Creation Date: | 25 June 2007 |
| Last Reviewed: | 12 March 2010 |