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UC Pest Management Guidelines


Pustules of rose rust appear as reddish brown spots on the lower leaf surface (left and bottom) and as yellow patches on the upper leaf surface (right).

Floriculture and Ornamental Nurseries

Rust

Pathogens: Puccinia spp. and others

(Reviewed 3/09, updated 3/09)

In this Guideline:


SYMPTOMS AND SIGNS

Rust pustules appear as powdery masses of yellow, orange, purple, black, or brown spores on leaves and sometimes on stems. Pustules are usually found on the undersides of leaves.

COMMENTS ON THE DISEASE

The rust fungi are obligate parasites in the order Uredinales. Many have complicated life cycles that include up to four different spore stages and one or possibly two distinctly different hosts. Other rusts produce all or sometimes only one type of spore and infect only one kind of plant. Stem rust (Puccinia graminis) of wheat alternates between barberry (Berberis spp.) and wheat (Triticum vulgare) and all four spore stages are produced. Rose rust also produces all four spore stages but only roses (Rosa spp.) are infected.

Urediniospores ("repeating" spores) are produced in pustules that appear yellow, orange, or brown as a result of masses of spores. Urediniospores are windborne and infect the plant through stomata. Water is required for short periods (6-8 hours or less) for germination and infection. Heavy dew is often sufficient. Once infection has occurred, water is no longer needed for continued development, and the infection and spore production will continue for the life of the leaf.

Some rusts, including rose rust (Phragmidium mucronatum), may survive the winter on leaves that do not fall off the plant. Rose rust also survives as teliospores (dark, overwintering spores) that form in fall. These spores survive in a dormant stage on fallen leaves. It is recommended that leaves with teliospores be removed, rapid composted, or buried.

MANAGEMENT

Rust diseases are favored by moderate temperatures that favor the growth of the host. Rust spores can be killed by high temperatures. Some rust infections, such as geranium rust (Puccinia pelargoni-zonalis), can be eradicated by hot water treatment of cuttings, but some damage to the host can occur.

Because water is necessary for infection, overhead irrigation should be avoided when rust is a problem. If possible, eliminate alternate hosts if they occur and prune off infected stalks. It is recommended that leaves with teliospores be removed, rapid composted, or buried.

Chrysanthemum white rust, caused by Puccinia horiana, is under an eradication program in California. If you have this disease contact your agricultural commissioner. See specific guidelines under the DISEASE CONTROL OUTLINE FOR CHRYSANTHEMUM.

There are several fungicides that can be used to protect plants from infection. Mancozeb as a protectant and myclobutanil as an eradicant are generally effective against all rusts while triadimefon is effective against only specific rusts.

Common name Amount to Use R.E.I.+
(trade name)   (hours)

  Calculate impact of pesticide on air quality
When choosing a fungicide, consider the general properties of the fungicide as well as information relating to environmental impact.
 
A. MANCOZEB
  (Dithane) 75W 1–1.5 lb/100 gal water 24
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: Provides protection only; must be applied before infection. Protects against leaf spots, Botrytis, rusts, and blight. Thorough coverage is important for control.
 
B. MYCLOBUTANIL
  (Hoist) 40WSP 4 oz/100 gal water 24
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: A systemic fungicide applied as a foliar spray; both a protectant and eradicant of rusts or powdery mildew on carnations, crepe myrtle, gerbera, roses, and snapdragons.
 
C. TRIADIMEFON
  (Strike, Bayleton) Label rates 12
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: A long-lasting systemic fungicide that provides for general control of some powdery mildews, some rusts, and leaf blight and spots in greenhouses and commercial nurseries. Because this material is closely related to growth retardant materials, it can have a toxic effect on certain plants, such as greenhouse roses.
 
D. AZOXYSTROBIN
  (Heritage) 1–4 oz/100 gal water 4
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Apply as a broadcast or banded spray targeted at the foliage or crown of the plant. A locally systemic fungicide that is effective against rusts.
 
E. WETTABLE SULFUR# 1–3 lb/100 gal water 24
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M2)
  COMMENTS: Use a wetting agent. Not as effective as other materials. Apply this material with caution when temperatures exceed 85°F.
 
F. CHLOROTHALONIL
  (Quali-Pro) 38.5% 23–32 fl oz/100 gal water 12
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
  COMMENTS: Provides protection only; must be applied before infection.
 
G. PROPICONAZOLE
  (Banner Maxx) 2-24 fl oz/100 gal 24
  MODE OF ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3)
  COMMENTS: Controls white rust of chrysanthemums but may cause phytotoxic symptoms on some cultivars.
 
H. NEEM OIL#
  (Triact) 70 Label rates 4
  COMMENTS: Registered for landscape and nursery ornamentals, neem has some protectant properties against rust, but is not effective for rust on rose. Apply on a 14-day schedule.
 
* Permit required from county agricultural commissioner for purchase or use.
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.
# Acceptable for use on organically grown ornamentals.
+ 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Diseases
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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