Pest & Disease Management - Downy mildews on field vegetable crops

These pathogens are characterised by sporulating on the under-surfaces of leaves with chlorotic and necrotic areas found on the upper surfaces, writes Geoffrey Dixon.

Effect of downy mildew on beetroot leaves - image: Geoff Dixon
Effect of downy mildew on beetroot leaves - image: Geoff Dixon

Hyaloperonospora parasitica on brassicas, Peronospora destructor on alliums, Peronospora farinosa f. sp. betae on red beet, Peronospora farinosa f. sp. spinaciae on spinach and Peronospora viciae on peas and broad beans. These pathogens are characterised by sporulating on the under-surfaces of leaves with chlorotic and necrotic areas found on the upper surfaces.

A wide range of field vegetable crops are attacked by these pathogens, including onion, garlic, leek, chives, shallots, beetroot, brassicas, peas, broad beans and spinach.

Cause

Microbes of the water mould group of oomycetes, onion downy mildew has increased with rising areas of overwintered crops. They invade foliage and parasitise leaf tissue cells, producing asexually reproductive structures through the stomata. They tolerate a wide temperature range (4-25 degsC) requiring high humidity.

Sexual oospores remain dormant in onion bulbs, crop residues and volunteer plants.

Beetroot plants and seed may be infected by P. farinose f. sp. betae. This pathogen is seed-borne, hence widely dispersed. It may overwinter in seed crops, weeds and volunteer plants.

Brassica downy mildew produces both asexual and sexual reproductive structures. There is some evidence that it is also seed-borne. Specialised strains are thought to exist that attack particular crops. For example, types invading radish are only weakly pathogenic to other brassicas.

Pea downy mildew can be a major problem, especially where crops are irrigated. It is seed-borne and overwinters as oospores left in soil after pea and bean crops. These are a particularly important source of primary infection, with naturally infested soils containing two-to-20 oospores per gram of soil. Disease development is rapid below 10 degsC with wind, rain and irrigation dispersing asexual spores.

Blue mould is generally classed as the most destructive spinach disease. It requires cool and moist conditions, which the spinach canopy provides, especially in baby-leaf cropping. It can be seed-transmitted and oospores are also found in leaf tissue.

Symptoms

Early symptoms on alliums are bleaching of leaf tips, chlorotic blotches and then fuzzy purple sporulation, often in concentric circles. Heavy infestations substantially diminish bulb size and yield quality. The systemic infection of bulbs leads to rotting in storage.

Beet downy mildew can infect at any growth stage, causing yellowing and distortion of the younger leaves. Systemic invasion leads to distortion of the growing point, resulting in spindly growth. The leaves become thickened and distorted, and the developing root may crack, leading to a darkened heart rot.

Normally, brassica downy mildew is first seen as yellowing patches on the upper surfaces of cotyledons, followed by white sporulation on the under-surfaces. Mature lesions may become bleached and papery. The pathogen may become systemic, causing discolouration to cauliflower and calabrese heads, and pod infection of seed inflorescences, with flower stalk distortion. Brussels sprouts may have darkened speckling and black lesions penetrating into the centre of the buds. Radish roots show an area of blackening, with scarring and cracking rendering them unsaleable.

Pea and bean downy mildew starts with seedlings showing stunting, typical leaf lesions on upper surfaces and grey-to-purple sporulation on under-surfaces. As plants mature, pods and tendrils become infested. Those of leafless cultivars can show severe infestations. This pathogen is capable of rapid systemic infection and ultimately invades seeds, from where it can spread between seasons.

Blue mould of spinach is initially seen as irregular light-green or dull-yellow patches on cotyledons and true leaves, from which sporulation is evident on the leaf under-surfaces. Leaves become curled, distorted and blighted.

Weather effects

Onion downy mildew requires temperatures of below 22 degsC and free moisture on the foliage for three hours. Spores germinate at 10-12 degsC. These are spread in air currents and can survive for three days before penetrating.

This pathogen survives overwinter as sexual oospores and mycelium in bulbs and the oospores in soil.

Beet downy mildew has a wider range of temperature and humidity requirements, and needs six hours of free moisture for germination.

Brassica downy mildew is similarly encouraged by cool, moist weather. Extended periods of rain or over-long irrigation associated with cool temperatures (10-15 degsC) encourage spread by water splash and subsequent rapid colonisation and symptom development.

In peas and broad beans, infections are frequently spread from foci from diseased seeds. Spread is encouraged as the canopy develops, providing damp conditions in the foliage.

Conditions encouraging disease in spinach are similar to those for beet crops. Growing dense crops of spinach for baby-leaf production provides ideal conditions for disease development and spread.

Soil conditions

Fertile, well-drained soils are required by all these crops, which then grow rapidly. Stresses caused by excessive water content in soil and poor crop nutrition encourage disease development.

Integrated disease management

Husbandry

Onion sets and bulbs should be inspected for signs of infection before planting. Very valuable sets and bulbs may be partially freed of the pathogen by heat treatment. Field sanitation involving the removal or ploughing in of diseased residues and crop rotation over a three to four-year period free from all allium crops diminishes the likelihood of infection.

Select fields with good air movement and plant or drill rows parallel to the prevailing wind. These recommendations also apply to beetroot and Swiss chard, brassica and legume crops. Brassica and legume crops should be irrigated early in the day so foliage can dry.

Beet, pea and broad bean downy mildew may be seed-borne. Seed treatment prevents the development of initial foci of infection in emerging crops. Brassica transplants should be inspected for signs of downy mildew before accepting from propagators.

Resistance

Resistant cultivars of peas, broad beans and spinach are available. Resistance must be managed by rotating cultivars with differing resistance genes. This limits the development and spread of physiological races of pathogens tolerant to the resistance genes. There are some suggestions of resistance to P. destructor in allium species and H. parasitica in brassica.

Fungicides

Brassicas (leaf and flower head)

Chorothalonil + Metalaxyl-M,

Metalaxyl-M (off label)

Fluopicolide + propamocarb hydrochloride (off label)

 

Brassicas (mustard)

Fluopicolide + propamocarb hydrochloride (off label)

Fenamidone + fosetyl aluminium (off label)

mandipropamid.

 

Brassicas (root)

Dimethomorph (off label)

Metalaxyl-M (off label)

Propamocarb hydrochloride (off label).

 

Brassicas (salad greens)

Azoxystrobin (off label – for baby leaf production)

Dimethomorph (off label)

Mancozeb (off label)

Fenamidone + fosetyl aluminium(off label)

Mancozeb + metalaxyl M (off label)

  

Spinach

Boscalid + pyraclostrobin (off label)

Dimethomorph (off label),

Fenamidone + fosetyl aluminium (off label)

mandipropamid,

Metalaxyl-M.(off label)

 

Beans (broad)

Chorothalanil + Metalaxyl-M,

Metalaxyl-M (off label).

 

Peas

Cymoxanil + fludioxonil + Metalaxyl-M.

 

Beet

Boscalid + pyraclostrobin (off label),

Dimethomorph (off label),

Fenamidone + fosetyl aluminium (off label);

Fluopicolide + propamocarb hydrochloride (off label),

Fosetyl-alluminium + propamocarb hydrochloride (off label),

Mandipropamid,

Metalaxyl-M (off label).

 

Stem and bulb vegetables (onions, leeks, garlic)

Azoxystrobin,

Azoxystrobin (off label)

Azoxystrobin + chlorothalonil

Benthiavalicarb-isopropyl + mancozeb (off label)

Dimethomorph + mancozeb (off label)

Dimethomorph + pyraclostrobin

Fluopicolide + propamocarb hydrochloride (off label)

Mancozeb

Mancozeb + Metalaxyl M (off label)

Mancozeb + metalaxyl M (useful control)

Metalaxyl M (off label)

Propiconazole (off label)

Warning

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


Before commenting please read our rules for commenting on articles.

If you see a comment you find offensive, you can flag it as inappropriate. In the top right-hand corner of an individual comment, you will see 'flag as inappropriate'. Clicking this prompts us to review the comment. For further information see our rules for commenting on articles.

comments powered by Disqus
Follow us on:
  • Facebook
  • LinkedIn
  • Twitter
  • Google +
Horticulture Jobs
More Horticulture Jobs