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Purpling and reddening of older leaves is a symptom of Phytophthora root rot.

Cole Crops

Phytophthora Root Rot

Pathogen: Phytophthora megasperma

(Reviewed 6/07, updated 11/08)

In this Guideline:


SYMPTOMS AND SIGNS

The external surfaces and internal tissues of infected roots are water-soaked and dark in color and are rotted. Leaves, especially older ones, first turn purple-red and later yellow and then wilt. The plant may be stunted, and with time, the entire plant wilts. The stem near the soil line may turn black and become soft.

COMMENTS ON THE DISEASE

Phytophthora root rot occurs on cauliflower, Brussels sprouts, and other crucifers in the coastal areas. It usually occurs only if cole crops are planted in poorly draining, fine-textured soils that are kept overly wet. Hence, root rot is most often found at low spots in the field or at the tail-end of irrigation runs. The fungus is a soil inhabitant that survives in the soil for long periods.

MANAGEMENT

Control is difficult, but soil management that improves drainage, such as planting high, well-drained beds, and carefully irrigating to avoid prolonged saturation of the soil, will reduce chances of infection. Because drought stress also makes plants susceptible to Phytophthora, ensure an even supply of moisture without major fluctuations to help suppress disease development. If a field has a history of Phytophthora disease problems, a treatment can be applied at planting.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
When choosing a pesticide, 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.
 
+ 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.
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 fufungicides 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: 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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