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Black rot on cabbage and other crucifers

This bacterial disease is common on Long Island and can be found most years, but occurs sporadically because sources of this pathogen are limited.  Potential initial sources for a farm are contaminated seed, infected seedlings grown elsewhere, and water or insects moving the bacteria from affected crop at a nearby farm.  Commercial cruciferous seed is routinely tested and treated for the black rot pathogen.  A rainstorm with wind or surface water runoff could move bacteria from a neighboring farm.  This bacterium can survive between crops in infested plant debris and infected cruciferous weeds.  Thus, each year black rot typically is found on farms where it occurred before.  On Long Island, black rot is most common in fall, especially when rain is frequent.  Temperature above 77 F is optimum for symptom development.  Soft rot bacteria invading leaf tissue killed by black rot can lead to extensive losses.  Black rot bacteria can multiply in plants without causing symptoms when temperature is low.  

The characteristic symptom of black rot is V-shaped lesion extending inwards from the leaf edge.  Initially it is dull yellow (first image below), then the center turns brown and becomes desiccated (second image below).  It is the result of infection through hydathodes (pores where veins end at the leaf edge).  Sometimes affected leaf tissue is associated with insect feeding or other wounds which provide a means for bacteria to enter the leaf (third image below).  Symptoms appear about 10-14 days after infection; longer under suboptimal temperatures.  As the disease progresses, most of the leaf blade can become affected (fourth image below).  This also occurs when infection is systemic which can occur when bacteria were in the seed or infected roots (fifth image below).  Diagnostic for black rot is black veins.  These can be seen in the lesions (second image below) and when the leaf is cut through the lesion (sixth image below), and also inside the petiole when bacteria have moved systemically (seventh image below).  Bacteria can be seen streaming out of infected veins by examining under a compound microscope the edge of leaf pieces cut from lesions (eight and ninth images below).

Management practices for black rot include selecting seed that has been tested and treated for the pathogen.  Select resistant varieties when possible.  Treating seed with hot water can be effective for bacteria on and also inside seed when the seed lot is not heavily contaminated.  It is best to have seed treated commercially when possible.  Plant where cruciferous crops have not been grown for at least three years during which time cruciferous weeds were controlled.  Inspect plants at least once a week for symptoms.  Apply fungicides (see next paragraph) using a boom sprayer when foliage is dry.  An airblast sprayer can disperse bacteria that are in any water that is present.  Bacterial diseases are managed best starting applications before symptoms are seen.  Do not work in fields when plants are wet as bacteria in water can be moved on workers and equipment.  Work in crops with no symptoms before affected crops.  Clean equipment after use in an affected crop.  Overhead irrigate when plants are dry; when dew is present is the worst time as bacteria can be in guttation water that exudes out of hydathodes along leaf margins.  Immediately after harvest, destroy crop debris such as by flail chopping and then incorporate. 

Fungicides labeled for managing black rot include Actigard, a plant activator.  Regalia acts similarly by boosting the plant’s defense mechanisms.  It is approved for organic production.  These need to be applied starting before infection to obtain the most benefit.  Other fungicides are also best used on a preventive schedule.  Copper fungicides are most commonly used for black rot.  There are several products including organic formulations. They differ in type of copper and in amount of metallic and biologically active copper.  Applying a mancozeb fungicide with copper (or using ManKocide) can increase copper activity.   Serenade is another fungicide labeled for black rot and approved for use on organically-produced crops.  

Additional information and photographs are at Vegetable MD Online. 

Please Note: The specific directions on pesticide labels must be adhered to — they supersede these recommendations, if there is a conflict. Any reference to commercial products, trade or brand names is forinformation only; no endorsement is intended.

 

Black rot on cabbage

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