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UC Pest Management Guidelines


Fruit scabs caused by apple scab infection.

Apple

Apple Scab

Pathogen: Venturia inaequalis

(Reviewed 8/06, updated 3/09)

In this Guideline:


SYMPTOMS AND SIGNS

Superficial, velvety dark-olive-to-black spots on fruits and leaves are the initial symptoms of apple scab. These fruit spots become scablike with age and tissues may become misshapen.

COMMENTS ON THE DISEASE

The fungus survives in dead leaves on the ground. Primary spores are discharged during spring rains and infect young leaves and fruits during prolonged moisture. Secondary spores may spread disease from established infections on trees. A lack of spring rains in most years reduces its importance in California in the southern Central Valley, but as a rule, this disease requires yearly treatments in coastal and foothill districts. Ornamental crab apples are also hosts. As plant parts mature and the weather gets warmer, susceptibility to this disease decreases, but pinpoint scab can occur during extended periods of moisture during summer.

MANAGEMENT

The main objective in scab management is the reduction or prevention of primary infections in spring. Extensive primary infections result in poor fruit set and make scab control during the season more difficult. If primary infections are successfully controlled, secondary infections will not be serious. The key to success in scab control is exact timing and full coverage. Wet periods, temperature, and relative humidity are important factors. Because scab control often is part of a combination treatment aimed at other diseases and insect control, choice of materials and timing are also extremely important.

A fall foliar fertilizer application of zinc sulfate and urea hastens leaf fall and speeds decomposition of fallen leaves. This in turn reduces the level of overwintering apple scab inoculum.

Organically Acceptable Methods
Treatments with lime and sulfur, sulfur alone, or with approved fixed copper products are organically acceptable.

Monitoring
Apple scab outbreaks can be predicted based on temperature and moisture conditions. The table below, derived from research by Mills and La Plante, gives hours needed at various temperatures under constantly wet conditions for primary spores (ascospores) to cause infection in spring. This system for forecasting scab and timing sprays has been validated for apple-growing regions in the northern areas of California, but not for the central and southern San Joaquin Valley where temperatures tend to be higher in spring than those in the table.

TABLE 1. Mills and La Plante Table.

HOURS OF WETTING REQUIRED FOR INFECTION
Average temperature
(degrees °F)
From primary inoculum (ascospores)
(hours)
78 13
77 11
76 9.5
61–75 9
60 9.5
57–59 10
55–56 11
54 11.5
52–53 12
51 13
50 14
49 14.5
48 15
47 17
46 19
45 20
44 22
43 25
42 30
33–41 *
*Not known

How to use the table: Figure the average temperature for the rain period by adding the maximum and the minimum temperatures and dividing by 2. If wet periods are intermittent, total their durations until there is a period of at least 6 hours of continuous dryness. You will need a wetness recorder to do this efficiently. If the dry period is sunny, and drying is quick and thorough, it is assumed that 6 hours after the trees have dried, the danger is passed. If drying is slow, and humidity remains high, then the 6-hour dry period is extended by a safety margin of 3 to 4 hours.

Treatment Decisions
Unless wetness periods are being monitored as outlined in the section above, apply protective fungicides at regular intervals beginning with green tip. Be sure to monitor wetness periods throughout the spring to insure that trees are adequately protected.

PERIOD OF EFFECTIVENESS1
FUNGICIDE When used as protectant
(days)
When used as kickback2
(hours)
Captan 10 0
Copper 7–10
Dithane 10 0
Flint 7–10 100
Lime sulfur 5–7 36
Procure 7–10 72
Rally 14 96
Rubigan 14 96
Scala 7–10
Sovran 7–10 96
Sulfur (micronized) 5–7 0
Topsin M 10 36–48
Vangard 7–10 48
Ziram 7–10 0
— Information not available.
1 Where range of days or hours is given, the difference reflects application rates –higher rates offer longer protection.
2 Eradicant fungicides have systemic action. Some are translocated within the host tissue and are able to kill the scab fungus up to a certain length of time after infection occurs. This is called the kickback period. Because kickback periods may change, always check the label for the most recent information. Kickback is calculated from the beginning of an infection period, as determined by the Mills and La Plante table.
Common name   R.E.I.+ P.H.I.+

(trade name)

Amount to Use

(hours)

(days)


  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy. When choosing a pesticide, also consider information relating to environmental quality. Not all registered pesticides are listed. Always read label of product being used.
 
A. FENARIMOL
  (Rubigan) 1EC 9–12 oz/acre 12 30
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: Do not apply between popcorn and full bloom. Do not apply more than 84 fl oz/acre/season.
   
B. MYCLOBUTANIL
  (Rally) 40WP 1.25–2 oz/100 gal water 24 14
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: Apply 400 gal/acre. Do not apply between popcorn and full bloom. During periods favorable for scab development, use high rate at a 7 day application interval. For postbloom control, can be used in combination with another protectant fungicide. Do not apply more than 5 lb/acre/season or graze or feed cover crops grown in treated areas to livestock. For application by ground equipment only.
   
C. TRIFLUMIZOLE
  (Procure) 50WS 2–4 oz/100 gal 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: Do not exceed 16 oz/acre/application.
   
D. TRIFLOXYSTROBIN
  (Flint) 2–2.5 oz/acre 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Alternate with a sterol inhibitor fungicide. Do not apply more than 2 consecutive applications before alternating. Do not apply more than 11 fl oz/acre/season.
   
E. PYRIMETHANIL    
  (Scala) SC 7–10 fl oz/acre 12 72
  MODE OF ACTION GROUP NAME (NUMBER1): Anilinopyrimidine (9)
  COMMENTS: Make application on 7 day or longer, depending on disease conditions. Do not apply more than 40 fl oz product/acre/crop.
       
F. KRESOXIM METHYL    
  (Sovran) 3.2–6.4 oz/acre 12 30
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Begin applications at 1/2 inch green or when conditions are conducive to disease development. Repeat at 7- to 10-day intervals. Do not apply more than 25.6 oz/acre/season.
       
G. THIOPHANATE METHYL
  (Topsin-M) 70WP 1–1.5 lb/acre 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
  COMMENTS: Apply at 5- to 10-day intervals from green tip through petal fall; continue at 7-to 14-day intervals as needed. Do not apply more than 4 lb product/acre/season.
   
H. CYPRODINIL
  (Vangard) WG 3–5 oz/acre 12 72
  MODE OF ACTION GROUP NAME (NUMBER1): Anilinopyrimidine (9)
  COMMENTS: Make applications on a 7- to 10-day interval. At the 3 oz (protectant) rate there is no kickback period of control, whereas at the 5 oz rate this material has a 48 hour kickback period.
   
I. CAPTAN 50WP 2 lb/100 gal water 4 days 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M4)
  COMMENTS: Apply 400 gal/acre. Do not apply in combination with, immediately before, or closely following oil sprays.
 
J. LIQUID LIME SULFUR# 2 gal/100 gal 48 0
  . . . PLUS . . .
  WETTABLE SULFUR# 4–5 lb/100 gal 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Use wettable sulfur alone after full bloom as lime sulfur may burn foliage if applied during warm weather. Lime sulfur is incompatible with most other pesticides. Check before use. Do not apply within 3 weeks of an oil application.
   
K. LIQUID LIME SULFUR# 2–3 gal/100 gal 48 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Lime sulfur is incompatible with most other pesticides. Check before use. May burn foliage if applied during warm weather. Do not apply within 3 weeks of an oil application.
   
L. MICRONIZED SULFUR#
  (Thiolux, Microthiol, etc.) Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Can be applied after bloom. Do not apply within 3 weeks of an oil application.
   
M. MANCOZEB
  (Dithane) DF 3.2 lb/acre 24 77
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Begin applications at 0.25- to 0.5- inch green tip and continue at 7- to 10-day intervals. Do not exceed 22 lb/acre of Dithane.
   
N. ZIRAM 76DF 1.5–2 lb/100 gal or 6–8 lb/acre 48 14
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Apply from pre-bloom through cover sprays as needed. Do not apply more than 32 lb/acre/year.
   
O. FIXED COPPER# Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
  COMMENTS: Apply at bud break using high label rate; later applications should be at low label rates. May cause russetting at rates over 0.5 lb/acre. Not all copper compounds are approved for use in organic production; be sure to check individual products.
   
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
# Acceptable for use on organically grown produce.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1,4,9,11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Apple
UC ANR Publication 3432
Diseases
W. D. Gubler, Plant Pathology, UC Davis
Acknowledgment for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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