How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

The fungus infects leaves and floral parts, causing distinctive white, raised pustules to form underneath the plant epidermis. These blisterlike pustules sometimes result in twisted, deformed growth of the stem, leaves, or flowers. When mature, the epidermis covering the pustule will rupture, releasing powdery white sporangia (a type of spore) that can be carried by winds or splashing water onto neighboring host plants. Severely infected leaves can wither and die.

COMMENTS ON THE DISEASE

The white rust pathogen of crucifers infects only plants in this host group, including arugula, bok choy, broccoli raab (rappini), Brussels sprouts, cabbage, cauliflower, Chinese cabbage, collards, Japanese or Mizuna-type mustards, radish, tah tsai, and turnip. However, economic damage is only found on the crucifer crops in which the leaves are marketed. If free moisture and cool temperatures are present, the sporangia germinate by producing several smaller motile spores (zoospores) that swim and enter susceptible young tissues. Because A. candida is dependent on cool, wet conditions, the disease is consistently more severe during winter and early spring months. In addition to sporangia, A. candida also produces a second type of spore, the oospore, that can resist drying conditions and enable the fungus to survive in a dormant state in soil or crop residue. The white rust pathogen exists in the form of distinct races.

MANAGEMENT

White rust resistant cultivars do not appear to be available for the host plants grown in California. Reducing leaf moisture by avoiding sprinkler irrigation will not prevent white rust, but keeping leaves dry may reduce disease severity. For sensitive crops such as arugula and rappini, avoid planting in fields having a history of white rust problems; soilborne oospores may result in severe disease. Fungicides may be appropriate in some situations on leafy crucifer crops. The same fungicides that control downy mildews are also effective against white rust.

Common name		Amount/Acre	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
The following materials are listed in order of usefulness in an IPM Program. Also, consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
A.	MEFENOXAM
	(Ridomil Gold) EC	1–2 pt	48	0
	MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)
	COMMENTS: Apply as a soil application at planting; can be preplant incorporated or applied as a soil surface spray after planting.
B.	MEFENOXAM/CHLOROTHALONIL
	(Ridomil Gold/Bravo) 76.5	1.5 lb	48	7
	MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M5) and phenylamide (4)
C.	FOSETYL-ALUMINUM
	(Aliette)	Label rates	12	3
	MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33)
	COMMENTS: Do not tank mix with copper compounds.
D.	COPPER#	Label rates	see label	0
	MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
	COMMENTS: Not all copper compounds are approved for use in organic production; be sure to check individual products.
+	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 http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode of action Group numbers 1, 4, 9, 11, or 17, make no more than one application 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.

PUBLICATION

UC IPM Pest Management Guidelines: Cole Crops
UC ANR Publication 3442
Diseases
S. T. Koike, UC Cooperative Extension, Monterey County
K. V. Subbarao, Plant Pathology,UC Davis, Salinas

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