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UC Pest Management Guidelines


Plum damaged by powdery mildew, Podosphaera sp.

Plum

Powdery Mildew

Pathogens: Sphaerotheca pannosa and Podosphaera tridactyla

(Reviewed 5/06, updated 4/09)

In this Guideline:


SYMPTOMS AND SIGNS

Areas of white powdery fungal growth, roughly circular in shape, develop on the fruit in spring. These infected areas later become scabby and dry. In late summer and fall, similar fungal growth appears on leaves. Occasionally, symptoms may develop on fruit and leaves in spring.

COMMENTS ON THE DISEASE

Sphaerotheca pannosa attacks the plum fruit whereas Podosphaera tridactyla attacks the foliage. An unidentified species, possibly of Podosphaera, attacks fruit and leaves of certain plum varieties (Red Beaut and Black Beaut) in spring; other varieties may be affected in some years as well.

Sphaerotheca pannosa is known to survive as mycelium on roses and in infected buds of peach trees, and these plants may serve as a source of inoculum for plum trees. This pathogen is not known to overwinter on plum, but recently cleistothecia were discovered on peach trees, which suggests that this pathogen may also produce cleistothecia and survive on plum trees.

Podosphaera tridactyla overwinters as special spore-forming structures called cleistothecia on the surface of shoots, on dead leaves on the orchard floor, and on bark. Spores are produced from these structures during spring rains, and they infect the developing foliage on plum trees. Growth of the pathogen is favored by cool, moist nights and warm days.

MANAGEMENT

Watching for the disease during routine monitoring helps to determine the need for possible action the following year, but by the time it appears on the fruit it is too late to spray during the current season. If there are roses infected with powdery mildew near the orchard, these bushes are potential sources of inoculum, and it may be beneficial to control the disease on the roses or to remove them.

Chemical Control
Apply a fungicide at full bloom and make additional applications on a 10- to 14-day interval as needed. The fruit is thought to be resistant to infection after pit hardening. It is important to alternate fungicides of a different chemistry to prevent the development of resistance.

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. Not all registered pesticides are listed. Always read label of product being used.
 
A. PROPICONAZOLE
  (Orbit) 3.6 EC 4 fl oz/acre 24 0
  (Bumper) 41.8 EC 4 fl oz/acre 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: A DMI-triazole fungicide. Do not apply to "Stanley" type plums. Do not apply more than 8 oz/acre/crop from early bloom through petal fall and 8 oz/acre/crop from petal fall to harvest.
 
B. THIOPHANATE METHYL
  (Topsin-M) 70W 8 oz/100gal 12 1
  MODE OF ACTION GROUP NAME (NUMBER1): Methyl benzimidazole (1)
  COMMENTS: A benzimidazole fungicide. Do not apply more than two applications before alternating with a fungicide that has a different mode of action.
 
C. PYRACLOSTROBIN/BOSCALID
  (Pristine) 10.5–14.5 oz/acre 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Carboxamide (7)
  COMMENTS: A strobilurin/carboxyanilide fungicide.
 
D. MYCLOBUTANIL
  (Rally) 40W 2.5–6 oz/acre 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: A DMI-triazole fungicide. Do not apply more than 2.75 lb/acre/season.
 
E. SULFUR DUST# 50 lb/acre 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Do not apply within 3 weeks of an oil application.
 
F. WETTABLE SULFUR# 5–10 lb/100 gal water 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Do not apply within 3 weeks of an oil application.
 
G. WETTABLE SULFUR# 24 0
  . . . PLUS . . .
  LIQUID LIME SULFUR# Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Do not apply within 3 weeks of an oil application.
 
+ 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: Plum
UC ANR Publication 3462
Diseases
W. D. Gubler, Plant Pathology, UC Davis
J. E. Adaskaveg, Plant Pathology, UC Riverside
K. R. Day, UC Cooperative Extension, Tulare County
Acknowledgment for contributions to Diseases:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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