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Lettuce Downy Mildew Treatment: Signs Of Lettuce With Downy Mildew

Lettuce Downy Mildew Treatment: Signs Of Lettuce With Downy Mildew


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By: Bonnie L. Grant, Certified Urban Agriculturist

Downy mildew in lettuce can affect both the appearance and the yield of a crop. It has serious implications in commercial growing because the disease spreads easily in certain environmental conditions. It affects the leaves of the plant, which, unfortunately, is the part we eat. Leaves are discolored and become necrotic, eventually progressing to the stem. Control methods for lettuce with downy mildew start with using resistant varieties and the use of fungicides.

What is Lettuce Downy Mildew?

Fresh, crisp lettuce is a year-round treat. A nicely made salad is a perfect start to any meal and usually features fresh lettuce. The vegetable is easy to grow, even in the home garden, but certain pests and diseases can wreak havoc on crops. One of these is downy mildew. What is lettuce downy mildew? It is a fungus that spreads easily in certain weather conditions and can be very hard to control. Crop losses are common and the spores that cause it can spread over long distances.

Downy mildew can affect lettuce at any stage of growth. It stems from the fungus Bremia lactucae. The spores of this fungus splash up onto plants with rain or are airborne. It was reported in Europe in 1843, but not known in the U.S. until 1875. Spores form during the night and are released during the day when humidity is lowered. A second generation of spores is produced within 5 to 7 days.

Between the prolific nature of the spores and the ease of spread, the disease can infect an entire crop in no time. Downy mildew in lettuce becomes epidemic in periods of cool weather with high daytime humidity.

Recognizing Lettuce with Downy Mildew

The early symptoms on seedlings are white cottony growth on the young plants followed by stunting and death. Older plants have the outer leaves affected first. They will display lighter green to yellow spots at the veins. Eventually, these become tan to brown and necrotic.

The white, fluffy growth is produced on the underside of the leaf. As the outer leaves become infected, the disease progresses to the interior leaves. If allowed to progress, the fungus will penetrate to the stem where stem rot occurs. The fungus also allows outside bacteria to infect the tissue, speeding up the deterioration of the head.

In mature plants that have only recently developed the fungus, the outer leaves can be removed and the head will usually be fine to consume.

Lettuce Downy Mildew Treatment

Control of the disease can be achieved by using resistant strains of lettuce seed. In commercial stands, both systemic and foliar fungicides are used but must be applied before any signs of the disease.

Irrigation systems that are set up to prevent wet leaves have excellent control, as does the provision of plenty of ventilation.

The timing of planting can also be important to effective lettuce downy mildew treatment. If possible, choose a time when ambient moisture isn’t at its height. Also, select an area in the garden that will dry quickly of nighttime dew.

Watch lettuce crops carefully for any sign of the fungus and treat or remove plants immediately.

This article was last updated on


Powdery Mildew vs Downy Mildew

When dealing with disease in the garden, it is always best to know what you’re up against before you start treating. Keeping symptoms straight can be confusing, and keeping names straight adds another layer of deduction. Downy mildew or powdery mildew—while both are called “mildew,” there are differences in how they look on your plants and how you treat the disease.

So how do you tell the difference at first inspection?

Powdery Mildew vs Downy Mildew
Powdery Mildew Downy Mildew
Fungal spots have a circular white appearance Fungal spots have an angular and gray appearance
Fungus can be anywhere on the leaf surface Fungus is limited by leaf veins
Leaves yellow after fungus has been present for a while Leaves may yellow before the presence of fungus is even evident


Look for the yellowish angular spots on the top of the older leaves, and white fungal-like growth on the underside. Look for spots that turn brown as they age.

QUARANTINE
As this disease is not recorded in many countries, biosecurity authorities should consider the potential pathways for entry.

  • Keep nurseries free from weeds, especially those in the daisy or sunflower family.
  • Avoid overcrowding seedlings. Make sure that there is sufficient air movement around them to dry leaves rapidly after watering or rain.
  • Carefully check each seedling before transplanting in the field, and remove any that show downy mildew symptoms. If symptoms are seen, spray all the seedlings with a systemic fungicide.

  • Space plants so that air can circulate around them this helps to dry the leaves rapidly after irrigation or rain.
  • Preferably, avoid overhead irrigation, especially during cool times. It is important that the foliage does not stay wet overnight, as this is the time that downy mildews produce their spores.
  • Control weeds. Weeds can be a source of downy mildew, and also increase the humidity around the crop so that leaves stay wet longer. The longer leaves stay wet the greater the chance of spore germination and infection.
  • Monitor plants for the first infections, and spray if weather is cool (less than 23°C) and damp (humidity more than 85%).

  • Collect plant remains, remove and burn or dig them deeply into the soil.
  • Do not plant lettuce in the same land if the crop was diseased leave a gap of 2-3 years. Oospores - the resistant spores - can survive in the soil.
  • Avoid planting overlapping crops of lettuce if downy mildew outbreaks occur. It is common for older crops to infect younger ones.

RESISTANT VARIETIES
Varieties with resistance to downy mildew exist, but there are several strains.

CHEMICAL CONTROL
Use fungicides that are recommended for Pythium and Phytophthora. Spray with systemic products, e.g., (i) metalaxyl (mixed with mancozeb or as alternating sprays to prevent build up of resistance to metalaxyl) or (ii) fosetyl-Al or use protectant products, e.g., mancozeb, copper or chlorothalonil. To increase effect of fungicides, keep humidity and leaf wetness as low as possible (see above).

AUTHOR Grahame Jackson
Information (and Photo 1) from Diseases of vegetable crops in Australia (2010). Editors, Denis Persley, Tony Cooke, Susan House. CSIRO Publishing and from CABI (2014) Bremia lactucae Crop Protection Compendium ( http://www.cabi.org.cpc/ ). Photos 2&3 Kohler F, Pellegrin F, Jackson G, McKenzie E (1997) Diseases of cultivated crops in Pacific Island countries . South Pacific Commission. Pirie Printers Pty Limited, Canberra, Australia.

Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production , implemented by the University of Queensland and the Secretariat of the Pacific Community.

This fact sheet is a part of the app Pacific Pests and Pathogens

The mobile application is available from the Google Play Store and Apple iTunes.


Eleven new major resistance genes for lettuce downy mildew were introgressed from wild Lactuca species and mapped to small regions in the lettuce genome. Downy mildew, caused by the oomycete pathogen Bremia lactucae Regel, is the most important disease of lettuce (Lactuca sativa L.). The most effective method to control this disease is by using resistant cultivars expressing dominant resistance genes (Dm genes). In order to counter changes in pathogen virulence, multiple resistance genes have been introgressed from wild species by repeated backcrosses to cultivated lettuce, resulting in numerous near-isogenic lines (NILs) only differing for small chromosome regions that are associated with resistance. Low-pass, whole genome sequencing of 11 NILs was used to identify the chromosome segments introgressed from the wild donor species. This located the candidate chromosomal positions for resistance genes as well as additional segments. F2 segregating populations derived from these NILs were used to genetically map the resistance genes to one or two loci in the lettuce reference genome. Precise knowledge of the location of new Dm genes provides the foundation for marker-assisted selection to breed cultivars with multiple genes for resistance to downy mildew.

Conflict of interest statement

The authors declare that they have no conflicts of interest.

Figures

Detection of introgression in Salinas…

Detection of introgression in Salinas 88. Detection of SNP variants against the Lactuca…

Example of introgressions in near…

Example of introgressions in near isogenic lines (NILs) that are co-located with MRCs.…

Graphic representation of lettuce Chromosomes…

Graphic representation of lettuce Chromosomes 1, 2, 3, and 4 with the genomic…

Virulence phenotypes and races of…

Virulence phenotypes and races of isolates of B. lactucae detected by the resistance…


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