Agriculture: Almond Pest Management Guidelines

Fruit Russeting (Powdery Mildew-Like)

Symptoms and Signs

Russeting on almond hulls is reminiscent of rusty spot on peach fruit caused by P. leucotricha, but the typical powdery white growth is absent. No conidia (asexual spores) or chasmothecia (sexual fruiting structures) of powdery mildew fungi have been observed. Only thin hyphae, not characteristic of powdery mildew fungi, are present. An Acremonium species has consistently been observed sporulating on the fruit. Foliar and twig symptoms are absent.

Comments on the Disease

The cause of fruit russeting of almond is unknown, but powdery mildew fungi (Podosphaera pannosa, P. tridactyla, and P. leucotricha) have been implicated. The absence of characteristic powdery mildew conidia and the presence of hyphae not typical for powdery mildew fungi provide evidence that different fungi are involved. More recently, an Acremonium species has been consistently observed growing and sporulating on, and has been isolated from, diseased fruit. Studies are currently being conducted to conclusively determine the pathogens involved.

Numerous fungicides with different modes of action are effective against the disease, including many that are effective against powdery mildew (e.g., demethylation inhibitor (DMI) and quinone outside inhibitor (QoI) fungicides) and others that are not (e.g., iprodione). The difficulty in working with powdery mildew fungi and Acremonium spp. has made absolute identification of the pathogen complicated.

The disease is only rarely reported in California and generally does not cause economic crop losses. Growth of the pathogen is favored by cool moist nights and warm days. Generally, fruit are susceptible only up to time of shell hardening, but later infections can occur on some varieties. Certain cultivars such as Mission, Padre, and Ruby are most susceptible.

Management

The disease is rarely an economic problem on almonds in California. If management is needed, control practices should focus on protecting fruit from infections. Jacket-split (late petal fall) and mid-spring applications of fungicides are highly effective in managing the disease.

Organically Acceptable Methods

Some sulfur sprays are acceptable for use in an organically certified crop.

Chemical Control

Generally, chemical control is not needed. If necessary, apply a fungicide from jacket split until mid-April.

Common name Amount per acre REI‡ PHI‡
(Example trade name) (hours) (days)
Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
 
A. MYCLOBUTANIL
  (Rally 40WSP, Laredo EW) Label rates 24 90
  MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation (sterol) inhibitor (3)
  COMMENTS: Do not make more than two successive applications and no more than three per season to limit the development of resistance. Use in rotation with different FRAC groups.
 
B. TEBUCONAZOLE
  (Toledo, Tebucon) 4–8 oz 12 35
  MODE-OF-ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3)
  COMMENTS: Do not make more than two successive applications and no more than four applications per season to limit the development of resistance.
 
C. PROPICONAZOLE
  (Bumper, Tilt) 8 oz 12 60
  MODE-OF-ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3)
  COMMENTS: Do not make more than two successive applications, and no more than four per season, to limit the development of resistance.
 
D. AZOXYSTROBIN/PROPICONAZOLE
  (Quilt Xcel) 14–26.0 fl oz 12 60
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and demethylation (sterol) inhibitor (3)
  COMMENTS: Do not make more than two successive applications, and no more than four applications per season, to limit the development of resistance.
 
E. PYRACLOSTROBIN/BOSCALID
  (Pristine) 10.5–14.5 oz 12 25
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and succinate dehydrogenase inhibitor (7)
  COMMENTS: Do not make more than four applications per season of QoIs or SDHIs and no more than two sequential applications before rotation to a different mode of action fungicide to limit the potential for the development of resistance.
 
F. PYRACLOSTROBIN/FLUXAPYROXAD
  (Merivon) 5–6.5 fl oz 12 14
  MODE-OF-ACTION GROUP NAME (NUMBER1): quinone outside inhibitor (11) and succinate dehydrogenase inhibitor (7)
  COMMENTS: Do not make more than four applications per season of QoIs (strobilurins) or SDHIs to limit the potential for the development of resistance.
 
G. METCONAZOLE
  (Quash) 3.5 oz 12 25
  MODE-OF-ACTION GROUP NAME (NUMBER1): demethylation (sterol) inhibitor (3)
  COMMENTS: Do not make more than two successive applications, and no more than four per season, to limit the development of resistance.
 
H. FLUOPYRAM/TEBUCONAZOLE
  (Luna Experience) 6.0–17.0 fl oz 12 35
  MODE-OF-ACTION GROUP NAME (NUMBER1): succinate dehydrogenase inhibitor (7) and demethylation (sterol) inhibitor (3)
  COMMENTS: Do not make more than two successive applications and no more than two per season to limit the development of resistance.
 
I. SULFUR DUST#
    Label rates See label 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Do not apply within 3 weeks of an oil application. Check with your certifier to determine which products are organically acceptable.
 
J. WETTABLE SULFUR#
    Label rates See label 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Do not apply within 3 weeks of an oil application. Check with your certifier to determine which products are organically acceptable.
 
K. WETTABLE SULFUR#
    Label rates See label 0
  . . . PLUS . . .
  LIQUID LIME SULFUR#
    Label rates See label 0
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Do not apply within 3 weeks of an oil application. Check with your certifier to determine which products are organically acceptable.
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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 action. 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 a fungicide with a different mode-of-action group number.
QoI = quinone outside inhibitor or strobilurin
SDHI = succinate dehydrogenase inhibitor
Text Updated: 08/17
Treatment Table Updated: 08/17