How to Manage Pests

UC Pest Management Guidelines

Black spots on fruit and leaf lesion from pear scab infection.

Pear

Pear Scab

Pathogen: Venturia pirina

(Reviewed 11/12 , updated 11/12 )

In this Guideline:


SYMPTOMS AND SIGNS

Scab first appears as velvety, dark olive-to-black spots on fruit, leaves, and stems. The olive velvety spots are the sporulating fungus and growth of the pathogen on the lesions. When infections occur early, fruit spots become scablike with age and the fruit may become misshapen. On leaves, infections cause leaf puckering and twisting and eventually tearing with age. Secondary infections that occur later in the season appear as black pinpoint spots on fruit and leaves.

COMMENTS ON THE DISEASE

In California, the fungus overwinters in dead leaves on the ground. Primary spores are discharged from flask-shaped fruiting bodies in infected dead leaves during spring rains and infect young leaves and fruit during periods of prolonged moisture. These infections produce secondary spores, which may cause further spread of disease during wet periods. Overwintering twig lesions from secondary infections may also be an important source of innoculum in spring.

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, such as by applying lime sulfur during the delayed-dormant season, or preventative fungicide treatments starting at green tip based on the Mills and La Plante Table output (Table 1), secondary infections should not be serious.

Resistance

Many of the fungicides, especially the new products used for controlling scab in pears, are single-site mode of action materials, meaning they stop or change only one metabolic step within the fungus. As these products are used over and over again in a season or in multiple seasons, the pathogen has the ability to develop resistance to them. Resistance to fungicides can occur in several ways. While the fungus can mutate on its own to develop resistance, selection pressure caused by the repeated use of one class of fungicides is usually the major cause. This has been documented with several fungicide modes of action—demethylation inhibitors (DMIs), benzimidazoles and the quinone outside inhibitors (QoIs)—in California's North Coast pear production areas. Other practices that often lead to resistance are the practices of using reduced concentrations of product and poor coverage; the latter is usually the result of tractor speeds that are too fast, reduced volumes of water, or making applications only to every other row.

Biological Control

No biological controls have been found.

Monitoring

Monitoring weather for potential pear scab outbreaks is the most important component of a control program. Pear scab outbreaks can be predicted based on temperature and moisture conditions. The table listed here, derived from research by Mills and La Plante, gives hours needed at various temperature levels under constantly wet conditions for primary spores to cause infection. In recent years automated weather stations have been programmed to provide this information. If wet periods are intermittent, add their durations until there is a period of at least 8 hours of continuous dryness. 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 has passed. If drying is slow and humidity remains high, then the 8-hour dry period is extended by a safety margin of 3 to 4 hours.

TABLE 1. Mills and La Plante Table: Hours of Wetting Required for Primary Infection During Spring. (After infection is established and secondary spores are present in large numbers, hours for reinfection are only two-thirds of figures shown.)
Average temperature (F) HOURS TO INFECT Days until lesions appear
Light infection Moderate infection Heavy infection
33–41 over 48
42 30 40 60
43 25 34 51
44 22 30 45
45 20 27 41
46 19 25 38
47 17 23 35
48 15 20 30 17
49 14.5 20 30 17
50 14 19 29 16
51 13 18 27 16
52 12 18 26 15
53 12 17 25 15
54 11.5 16 24 14
55 11 16 24 14
56 11 15 22 13
57 10 14 22 13
58 10 14 21 12
59 10 13 21 12
60 9.5 13 20 11
61 9 13 20 10
62 9 12 19 10
63 9 12 18 9
64 9 12 18 9
65 9 12 18 9
66–75 9 12 18 8
76 9.5 12 19 9
77 11 14 21 9
78 13 17 26 10
Harvest fruit sample

At harvest, assess your IPM program by monitoring fruit in the bins for pear scab damage. Sample 200 fruit per bin from 5 bins per orchard (or 20-acre block in large orchards). For more information, see HARVEST FRUIT SAMPLE.

Postharvest shoot sample

Following harvest, sample 20 top shoots for scab lesions on the leaves. If lesions are found, make plans to treat in spring. For more information about sampling, see POSTHARVEST SURVEY and DORMANT/DELAYED-DORMANT SAMPLING.

Treatment Decisions
Preventive program

Apply high rates (6 gal/100 gal water with up to 24 gal/acre) of lime sulfur in the delayed-dormant period to significantly reduce the viability of overwintering twig lesions, which can be a significant source of inoculum during and in years following severe scab outbreaks.

Use a preventive program in conjunction with the Mills and La Plante table, particularly:

  • During high-rainfall years
  • If scab was problem last year
  • For areas with heavy rainfall

If moisture and temperature predict conditions conducive to pear scab, treat every 7 to 10 days starting at green tip until primary spore release is complete (5 to 10 weeks). Thereafter, maintain vigilance until hot, dry weather produces conditions unfavorable for infection.

Eradicant program

In most cases, an eradicant program uses single-site-of-action fungicides, and so resistance development is an issue. If needed, switch to an eradicant program after bloom to reduce the risk of resistance development. For low-rainfall areas that only require occasional treatments, only eradicant treatments may be necessary.

Treat as soon as possible after an infection period with a material that has kickback control of established infections. Thorough coverage is very important; apply eradicant fungicides to every row. See the table below ("Period of Effectiveness") for a list of fungicides with kickback activity. Treatments will be effective only for the stated period of time after the beginning of the infection period, not after symptoms of the disease have appeared.

Monitor infection periods daily throughout the spring and use the Mills and La Plante table to ensure coverage of newly exposed growth as long as rains are occurring frequently. Where infections are severe, consider applying fall and/or delayed-dormant treatments of lime sulfur to reduce overwintering inoculum.

Fungicide1 PERIOD OF EFFECTIVENESS2
When used as protectant (days) When used as kickback3 (hours)
Copper 7–10
Flint4 7–10 72–96
Inspire Super 7–10 48
Lime sulfur 5–7 36
Manzate 7–10 0
Pristine 7–10 0
Procure 7–10 48–96
Rubigan 14 96
Scala 7 05
Sovran 7–10 96
Syllit 5–7 36
Topsin-M 5–10 36–48
Vangard 7–10 05
Vintage 7 96
Wettable sulfur 3–5 (or until rained on) 0
Ziram 7 0
 
1 Do not use selected demethylation inhibitors (DMIs), such as Rubigan and Vintage, from popcorn to full bloom. Procure can be used prior to petal fall. Higher rates and reduced intervals between applications have been suggested for Procure when disease pressure is high.
2 Where range of days or hours is given, the difference reflects application rates—higher rates offer longer protection. Appearance of new, unprotected growth will often dictate the length of the effectiveness period in relation to infection periods.
3 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 action is calculated from the beginning of an infection period as determined by the Mills table. Frequent use of products as a kickback material may lead to the development of resistance, except in the case of lime sulfur.
4 Flint can not be used in the North Coast due to resistance.
5 This material has no kickback period at the rate that it is registered for use on pears.
No information.
 
Common name Amount to use** R.E.I.+ P.H.I.+
(trade name)   (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. Different formulations with the same active ingredient may contain inert ingredients that can affect performance; specific labels should be followed accordingly. Not all registered pesticides are listed. Always read the label of the product being used.
 
DELAYED-DORMANT
 
A. LIME SULFUR# 6 gal/
100 gal water
48 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Highly effective at killing twig lesions; apply lime sulfur at this time if twig lesions are present. Apply in 100 to 400 gal/acre. Lime sulfur is incompatible with most other pesticides. Check before use. Do not combine with oil or use on Comice.
 
GREEN TIP THROUGH SPRING
 
A. FENARIMOL
  (Rubigan EC) 9–12 oz/acre 12 30
  (Vintage SC) 9–12 oz/acre 24 30
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: Follow label directions carefully. Use only before white bud and after full bloom. Has protectant and eradicant action.
 
B. TRIFLUMIZOLE
  (Procure 480SC) 8–16 oz/acre 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: Use only before white bud and after full bloom. Has protectant and eradicant action.
 
C. DIFENOCONAZOLE + CYPRODINIL
  (Inspire Super) 12 oz/acre 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) and Anilinopyrimidine (9)
  COMMENTS: Has protectant and eradicant action. Starting at green bud, apply in 7- to 10-day intervals. Use shorter intervals under high disease pressure. Do not apply more than two sequential applications before rotating to a different mode of action and no more than four applications per season.
 
D. TEBUCONAZOLE
  (Tebuzol 45DF) 4–8 oz/acre 120 (5 days) 75
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: Has protectant and eradicant action. Starting at green bud, apply in 7- to 10-day intervals. Use shorter intervals under high disease pressure. Do not apply more than two sequential applications before rotating to a different mode of action and no more than 3 lb a.i. per season.
 
E. TEBUCONAZOLE + TRIFLOXYSTROBIN
  (Adament WG) 4–5 oz/acre 12 75
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) and quinone outside inhibitor (11)
  COMMENTS: Has protectant and eradicant action. Starting at green bud, apply in 7- to 10-day intervals. Use shorter intervals under high disease pressure. Do not apply more than two sequential applications before rotating to a different mode of action and no more than four applications per season.
 
F. TRIFLOXYSTROBIN
  (Flint) 2–2.5 oz/acre 12 14
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not apply more than 2 consecutive applications before alternating with a fungicide that has a different mode of action. May not be used in North Coast orchards because of resistance problems.
 
G. PYRACLOSTROBIN + BOSCALID
  (Pristine WG) 14.5–18.5 oz/acre 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not apply more than two consecutive applications before alternating with a fungicide that has a different mode of action. Resistance problems may occur in areas with QoI resistance.
 
H. CYPRODINIL
  (Vangard WG) 3–5 oz/acre 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Anilinopyrimidine (9)
  COMMENTS: Apply as a tank mix only with fungicides of a different chemistry (e.g., Inspire Super). Do not apply cyprodinil by itself to pears. This material is more effective when temperatures are below 80°F. Do not apply more than 30 oz/acre per year.
 
I. 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 per season.
 
J. PYRIMETHANIL
  (Scala SC) 7–10 fl oz/acre 12 72
  MODE OF ACTION GROUP NAME (NUMBER1): Anilinopyrimidine (9)
  COMMENTS: Make application on a 7-day interval or longer, depending on disease conditions. This material is more effective when temperatures are below 80°F. Do not apply more than 40 fl oz product/acre per crop.
 
K. THIOPHANATE METHYL
  (Topsin-M, t-Methyl, Incognito) Label rates see label 1
  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 per season. Toxic to earthworms with repeated usage.
 
L. MANCOZEB
  (Dithane DF, Penncozeb, Manzate) 3–6 lb/acre 24 77
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Apply according to label instructions. This material is used when russeting is a concern. The high rate is necessary for good scab and russet control.
 
M. DODINE
  (Syllit FL) 3 pt/acre 48 7
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M7)
  COMMENTS: Apply at 5- to 7-day intervals or as needed. Do not apply less than 7 days from last application. Do not make more than 3 applications per season or apply more than 9 pt (4 lb a.i.)/acre per year. A maximum of two applications may be made in the final two months before harvest: one made no later than 14 days before harvest and the final application no later than 7 days before harvest. Tank mix with another mode of action (such as propiconazole, tebuconazole, etc.) to slow resistance development.
 
N. LIME SULFUR# 2 gal/
100 gal water
48 0
  . . . PLUS . . .
  WETTABLE SULFUR# 4 lb/
100 gal water
24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Rates per 100 gal of water vary with manufacturer; always check label for recommended rate. Kills twig lesions. Do not apply after cluster bud. Sulfur used before bloom reduces crop set in Anjou and Comice. It is phytotoxic to Comice and should not be used on this variety. Caution: Do not apply within 3 weeks of an oil application. Lime sulfur is incompatible with most other pesticides; check before use.
 
O. LIME SULFUR# 2 gal/
100 gal water
48 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Kills twig lesions. Do not apply after cluster bud. Sulfur used before bloom reduces crop set in Anjou and Comice. It is phytotoxic to Comice and should not be used on this variety. Caution: Do not apply within 3 weeks of an oil application. Phytotoxicity may occur any time weather is hot so watch weather conditions closely. Lime sulfur is incompatible with most other pesticides; check before use.
 
P. ZIRAM 76DF 6 lb/acre 48 14
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Important in a resistance management program. Do not apply more than 4 applications or more than 24 lb/acre per crop cycle.
 
POSTHARVEST
 
A. LIME SULFUR# 3–5 gal/
100 gal water
48 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Do not apply any sooner than November 1 and only on trees not suffering from moisture stress. Phytotoxicity may occur anytime weather is hot, so watch weather conditions closely. Lime sulfur is incompatible with most other pesticides. Check before use. Apply with or without oil (5 gal/acre) but do not apply with oil before November 15. When applied with oil, maximum lime sulfur rate per acre is 5 gal.
 
** See label for dilution rate.
+ 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 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 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: Pear
UC ANR Publication 3455

Diseases

R. B. Elkins, UC Cooperative Extension, Lake County
W. D. Gubler, Plant Pathology, UC Davis
J. E. Adaskaveg, Plant Pathology, UC Riverside

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