Pest & Disease Management - Leptosphaeria maculans (Phoma leaf spot) on field vegetable crops

Disease profile for field vegetable growers, by Professor Geoffrey Dixon.

Leptosphaeria (Phoma leaf spots) with Pycnidia - Image: Yongju Huang
Leptosphaeria (Phoma leaf spots) with Pycnidia - Image: Yongju Huang

Leptosphaeria maculans (Phoma lingam): black leg - also known as Phoma leaf spot, canker and dry rot, especially in root crop brassicas.

 

Crops affected

All brassicas are vulnerable to this pathogen, which is considered one of the most important causes of losses worldwide in vegetable, arable and forage crops, especially oil seed rape. This provides a reservoir of infection that will spread into adjacent horticultural crops.

 

Causes

The asexual stage, Phoma lingam, is most frequently found on leaves as lesions or cankers containing massed dark-brown to black minute fruiting bodies (pycnidia), which ooze spores (conidia) in wet conditions and are spread by rain splash and less easily by wind currents.

In very humid weather, a white fluffy mycelium grows around the edges of cankers and this can be a useful diagnostic symptom. As the disease advances, a sexual stage (Leptosphaeria maculans) may form that is capable of destroying woody tissues. Biologically this pathogen is thought to consist of numerous distinct strains. Molecular studies are revealing individual and collective capacities for causing disease.

 

Symptoms

Normally first seen as circular to oval leaf spots 0.5-3cm in diameter that have a yellow halo. Leaf veins crossing the lesions turn black. Leaf spots vary in colour from greenish-brown in the early stages through pale-brown, tan and eventually off-white. Numerous minute asexual reproductive bodies (pycnidia) form in these lesions that en masse appear as black pin heads.

The pathogen spreads by penetrating through leaf veins into petioles and eventually enters the main stem. Here typical black leg symptoms become apparent as large, sunken, elongated, brown to black cankers. Stem tissues within the cankers are destroyed, impairing water movement and leading to wilting, stunting and unthrifty growth of the whole plant. Badly diseased stems become completely rotten and break. 

Infected seedlings develop foliar and stem lesions, fail to mature and die off. Affected maturing root crops such as swede and turnip develop dry rot of the bulb (hypocotyl), which carries large brown to black cankers on the upper neck and shoulders. Dry rotting extends deeply into the bulbs of these crops, which shrivel and are rendered valueless.

 

Weather effects

This pathogen spreads by rain-splashed asexual spores (conidia) in the canopies of affected crops and by airborne sexual ascospores that are transmitted more than a kilometre. The relative importance of these routes varies between crops and geographical localities.

Successful ascospore infection requires temperatures between 15 degsC and 32 degsC as well as a minimum of four hours of leaf wetness - preferably as long as 48 hours. The pathogen invades leaf tissues via stomata and areas of damage.

Aggressive strains of L. maculans appear capable of direct infection, penetrating through the leaf surfaces. Symptoms begin appearing three-to-four days after infection at 20 degsC and may take up to 14 days in cooler weather conditions.

 

Soil conditions

This pathogen is airborne and seed-borne, hence the incidence of epidemics is less influenced by soil conditions. Nonetheless, factors such as impaired drainage, excessive irrigation and unnecessary applications of nitrogenous fertilisers cause physiological stresses, which predispose crops to infection and disease development.

 

Integrated disease management

Resistant cultivars No resistant cultivars have been bred, but crops such as kale, collards, some swede and turnip cultivars, Chinese mustard, cress and horseradish are considered less susceptible.

 

Husbandry measures

This pathogen can be seed-borne and it is imperative that only high-grade certified seed is used. Regrettably, oil seed rape crops may be grown from farm-saved seed or self-established crops. Consequently they form a significant reservoir of infection capable of invading adjacent horticultural crops. 

Care should be exercised when planning horticultural cropping to avoid proximity to oil seed rape. The dangers posed by oil seed rape should be borne in mind when renting land for horticultural cropping. Once land has been cropped with infected oil seed rape the resultant debris will carry this pathogen in a viable form for three years even when deeply buried by ploughing. 

Crop rotation

Where feasible, this is an excellent means of minimising disease risk, especially if break crops such as agricultural legumes (clover or lucerne/alfalfa) can be used. The stand density of vulnerable crops such as Brussels sprouts, European cabbages and Chinese cabbage should be reduced to encourage air movement and avoid the build-up of moist conditions in the canopy. 

Stresses imposed on horticultural crops encourage this disease - herbicide damage in particular encourages disease development by offering portals for invasion.

Regular crop monitoring ensures that infection is identified early and allows remedial action to be taken.

 

Fungicides

Currently allowable chemical treatments include:

 

Brassicas – leaf and flower

Phoma leaf spots

Tebuconazole + trifloxystrobin

Stem canker

Cyprodinil + fludioxonil (off label)

Prothioconazole

 

Mustard

Stem canker

Cyprodinil + fludioxonil (off label)

 

Root brassicas

Leaf spot

Tebuconazole + trifloxystrobin (off label)

 

Salad greens

Stem canker

Cyprodinil + fludioxonil (off label)

Warning

For use on all fresh-produce vegetable crops growers must in advance of use ensure that a particular commercial agrochemical formulation is legally acceptable for their particular crop/husbandry regime and also accepted by the intended purchaser's crop-quality standards specification as agreed with the relevant crop technologist.


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Professor Geoffrey Dixon

GreenGene International chair Geoff Dixon on the business of fresh produce production
 

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