Agriculture: Nectarine Pest Management Guidelines

Rust

  • Tranzschelia discolor
  • Symptoms and Signs

    Infections of young twigs and leaves are the most common symptoms of rust, but in California, fruit infections may be a major component of the disease as well.

    Twig cankers

    Twig cankers are the first symptoms of the disease in spring. Cankers develop after petal fall on 1-year-old fruiting wood. They appear as blisters and longitudinal splits in the bark about 0.12 to 0.25 inches long. They can most easily be seen using a 20X hand lens.

    Leaf lesions

    Leaf lesions usually develop after cankers form in spring and may continue to develop through summer and into fall. The lesions appear as bright yellow, angular spots on the upper surface of leaves. The lower surface of the leaves contains brown spore masses. A high incidence of early leaf infections may cause midseason defoliation and numerous fruit infections at harvest. Early and severe defoliation also may reduce yields and stimulate the production of new leaves and buds late in the growing season.

    Fruit lesions

    Fruit lesions may develop during the growing season after leaf symptoms. They first develop as small, brownish spots (0.1 inch) with green halos on mature, yellow fruit. When fruit reddens, lesion halos become greenish yellow. The lesions are sunken and extend several millimeters into the fruit.

    Comments on the Disease

    Transchelia discolor survives in twig cankers or on other host parts, and airborne spores depend on wetness for infection. Fruit symptoms may resemble damage caused by stink bugs; confirm rust by the presence of rust spores within the fruit lesion or by leaf or twig symptoms.

    Management

    In orchards where rust develops it is managed with a fungicide treatment in spring. If the problem was severe the previous year, several fungicide treatments may be necessary in spring as soon as the trees leaf out. Because damp conditions favor rust development, angle sprinklers to avoid wetting the foliage. Drip irrigation is the least favorable to development of this disease because it doesn't increase the humidity in the orchard as much as flood or furrow irrigation.

    Take fruit samples every other week after color break (see PREHARVEST FRUIT SAMPLES) to detect any developing problems in the orchard and a fruit damage sample at harvest to assess the effectiveness of the current year's IPM program and to determine the needs of next year's program, see FRUIT EVALUATION AT HARVEST. Record results (PDF) for harvest sample.

    Organically Acceptable Methods

    Sulfur treatments are acceptable for use in an organically certified crop.

    Chemical Control

    To be effective, treatments must be applied before rust symptoms appear on leaves. Examine one-year-old fruiting wood for small blisters or longitudinal splits. If twig cankers are found and rain is forecasted, make a treatment. If wet weather persists, additional applications may be necessary in late May or early June. Disease severity in the preceding year is an important factor in determining potential of disease during current year.

    Treatment with sulfur is both cost-effective and efficacious. The sterol inhibitors are also efficacious but more expensive than the sulfurs.

    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.
     
    SULFUR TREATMENTS
    A. WETTABLE SULFUR# 18 lb/100 gal water See label See label
      MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
      COMMENTS: Do not apply within 3 weeks of an oil application.
     
    B. WETTABLE SULFUR+LIQUID LIME SULFUR# Label rates See label See label
      MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
      COMMENTS: Do not apply within 3 weeks of an oil application.
     
    C. LIQUID LIME SULFUR# 6 gal/100 gal water See label See label
      MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
      COMMENTS: Do not apply within 3 weeks of an oil application.
     
    D. SULFUR DUST# 50 lb See label See label
      MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
      COMMENTS: Do not apply within 3 weeks of an oil application.
     
    STEROL INHIBITOR FUNGICIDES
    A. TEBUCONAZOLE
      (Elite 45WP) 5–8 oz 5 days 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
      COMMENTS: Do not apply more than 3 lb/acre per season.
     
    B. PROPICONAZOLE
      (Bumper, Tilt) 4 oz 12 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
      COMMENTS: Maximum of 2 preharvest sprays.
     
    C. METCONAZOLE
      (Quash) 3.5–4 oz 12 14
      MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
      COMMENTS: Do not make more than 3 applications per season.
     
    STROBILURIN FUNGICIDE
    A. AZOXYSTROBIN
      (Abound) 12–15.5 fl oz 4 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
      COMMENTS: Do not apply more than two applications before alternating with a fungicide that has a different mode of action group number.
     
    B. TRIFLOXYSTROBIN
      (Gem 500 SC) 4–8 oz 12 1
      MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
    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 actions. 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.

    Important Links

    Text Updated: 06/10
    Treatment Table Updated: 09/15