Editing Lettuce (section)

=== Cultivation problems ===
[[File:Starr 081031-0356 Lactuca sativa.jpg|thumb|upright|A lettuce surrounded by weeds, which have crowded it to the point of [[bolting (horticulture)|bolting]]]]
{{see also|List of lettuce diseases}}

Soil nutrient deficiencies can cause a variety of plant problems that range from malformed plants to a lack of head growth.<ref name="Bradley129" /> Many insects are attracted to lettuce, including [[cutworm]]s, which cut seedlings off at the soil line; [[wireworm]]s and [[nematode]]s, which cause yellow, stunted plants; [[tarnished plant bug]]s and [[aphid]]s, which cause yellow, distorted leaves; [[leafhopper]]s, which cause stunted growth and pale leaves; [[thrip]]s, which turn leaves gray-green or silver; [[leafminer]]s, which create tunnels within the leaves; [[flea beetle]]s, which cut small holes in leaves and [[caterpillar]]s, [[slug]]s and [[snail]]s, which cut large holes in leaves. For example, the larvae of the [[ghost moth]] is a common pest of lettuce plants.<ref name="Edwards64">{{Cite journal|last=Edwards|first=C. A.|date=May 1964|title=The bionomics of swift moths. I.—The ghost swift moth, Hepialus humuli (L.)|journal=Bulletin of Entomological Research|volume=55|issue=1|pages=147–160|doi=10.1017/S000748530004935X|issn=1475-2670}}</ref> Mammals, including [[rabbit]]s and [[groundhog]]s, also eat the plants.<ref name="Bradley132">Bradley, pp. 129–132.</ref> Lettuce contains several defensive compounds, including [[sesquiterpene lactone]]s, and other natural [[phenols|phenolics]] such as [[flavonol]] and [[Flavonols#Flavonols glycosides|glycosides]], which help to protect it against pests. Certain varieties contain more than others, and some selective breeding and genetic modification studies have focused on using this trait to identify and produce commercial varieties with increased pest resistance.<ref>{{cite web|url=http://www.reeis.usda.gov/web/crisprojectpages/198636.html|title=Developing Multi-Species Insect Resistance in Romaine Lettuce|publisher=United States Department of Agriculture|access-date=5 April 2012|archive-url=https://web.archive.org/web/20131019193634/http://www.reeis.usda.gov/web/crisprojectpages/198636.html|archive-date=19 October 2013|url-status=dead}}</ref>

Lettuce also suffers from several [[Plant virus|viral diseases]], including [[Lettuce big-vein disease|big vein]], which causes yellow, distorted leaves, and [[lettuce mosaic virus|mosaic virus]], which is spread by aphids and causes stunted plant growth and deformed leaves. [[Aster yellows]] are a disease-causing bacteria carried by leafhoppers, which causes deformed leaves. Fungal diseases include [[powdery mildew]] and [[downy mildew]], which cause leaves to mold and die and [[Fungicide use in the United States#Bottom rot|bottom rot]], [[Fungicide use in the United States#Lettuce drop|lettuce drop]] and [[gray mold]], which cause entire plants to rot and collapse.<ref name="Bradley132" /> Bacterial diseases include ''[[Botrytis cinerea]]'', for which [[UV-C]] treatments may be used: Vàsquez et al. 2017 find that [[phenylalanine ammonia-lyase]] activity, phenolic production, and ''B. cinerea'' [[plant disease resistance|resistance]] are increased by UV-C.<ref name="Urban-et-al-2018">{{cite journal | last1=Urban | first1=L. | last2=Chabane Sari | first2=D. | last3=Orsal | first3=B. | last4=Lopes | first4=M. | last5=Miranda | first5=R. | last6=Aarrouf | first6=J. | title=UV-C light and pulsed light as alternatives to chemical and biological elicitors for stimulating plant natural defenses against fungal diseases | journal=Scientia Horticulturae | publisher=[[Elsevier]] | volume=235 | year=2018 | issn=0304-4238 | doi=10.1016/j.scienta.2018.02.057 | pages=452–459 | bibcode=2018ScHor.235..452U | s2cid=90436989}}</ref> Crowding lettuce tends to attract pests and diseases.<ref name="Weaver175" /> Weeds can also be an issue, as cultivated lettuce is generally not competitive with them, especially when directly seeded into the ground. Transplanted lettuce (started in flats and later moved to growing beds) is generally more competitive initially, but can still be crowded later in the season, causing misshapen lettuce and lower yields. Weeds also act as homes for insects and disease and can make harvesting more difficult.<ref>{{cite web|url=http://www.ipm.ucdavis.edu/PMG/r441700111.html|title=Lettuce: Integrated Weed Management|work=UC Pest Management Guidelines|publisher=University of California|date=August 2007|access-date=3 April 2012|archive-url=https://web.archive.org/web/20120707190840/http://www.ipm.ucdavis.edu/PMG/r441700111.html|archive-date=7 July 2012|url-status=dead}}</ref> Herbicides are often used to control weeds in commercial production. However, this has led to the development of [[herbicide-resistant]] weeds in lettuce cultivation.<ref name="Davey222" />