Disease management is a principle activity for growers of all types of lettuce. Viral, bacterial, and most importantly, fungal diseases can seriously impact quality and yield.

Arizona Lettuce Diseases

Disease management is a principle activity for growers of all types of lettuce as viral, bacterial, and most importantly, fungal diseases can seriously impact quality and yield. Many management strategies are based on chemical applications as these methods often provide the only level of control acceptable to the industry.

As such, pesticide applications on lettuce rank among the highest of any major vegetable crop. For example, fungicide applications in California in 2003 (metham-sodium not included) averaged 11.62 lbs/acre treated for head lettuce and 12.45 lbs/acre treated for leaf lettuce, for a total of 454 tons of fungicide statewide (Pesticide Action Network North America 2003. California Pesticide Use).

Major Lettuce Diseases in Arizona

Lettuce Drop
Fusarium Wilt
Gray Mold (Botrytis)
Powdery Mildew
Downey Mildew
Big Vein

Complete Lettuce Disease List

Source: American Phytopathological Society (with minor modifications)

Bacterial Diseases

Bacterial leaf spot and head rot
Xanthomonas campestris pv. vitians (Brown) Dye
Bacterial soft rot
Erwinia carotovora (Jones) Bergey et al.
Pseudomonas marginalis (Brown) Stevens
Corky root
Rhizomonas spp.
R. suberifaciens van Bruggen et al.
Varnish spot
Pseudomonas cichorii (Swingle) Stapp

Fungal Diseases

Alternaria leaf spot
Alternaria sonchi J.J. Davis in J.A. Elliott
Anthracnose
Microdochium panattonianum (Berl.) Sutton et al. in Galea et al.
Marssonina panattoniana (Berl.) Magnus
Bottom rot
Rhizoctonia solani Kühn
teleomorph: Thanatephorus cucumeris (A.B. Frank) Donk)
Cercospora leaf spot
Cercospora longissima Sacc.
Damping-off, Pythium
Pythium spp.
P. ultimum Trow
Damping-off, Rhizoctonia
Rhizoctonia solani Kühn
Downey mildew
Bremia lactucae Regel
Plasmopara lactucae-radicis Stanghellini and Gilbertson
Drop (Sclerotinia rot)
Sclerotinia sclerotiorum (Lib.) de Bary
S. minor Jaggar
Fusarium Wilt
Gray mold
Botrytis cinerea Pers.:Fr.
(teleomorph: Botryotinia fuckeliana (de Bary) Whetzel)
Phymatotrichum root rot (cotton root rot)
Phymatotrichopsis omnivora (Duggar) Hennebert
= Phymatotrichum omnivorum Duggar
Powdery mildew
Erysiphe cichoracearum DC.
Rust
Puccinia dioicae Magnus
= P. extensicola Plowr. var. hieraciata (Schwein.) Arthur
Septoria leaf spot
Septoria lactucae Pass.
Southern blight
Sclerotium rolfsii Sacc.
(teleomorph: Athelia rolfsii (Curzi) Tu and Kimbrough)
Stemphylium leaf spot
Stemphylium botryosumWallr.
(teleomorph: Pleospora tarda E. Simmons)
Verticillium Wilt
Wilt and leaf blight
Pythium tracheiphilum Matta

Miscellaneous Diseases or Disorders

Brown stain
Physiological, excess CO2
Russet spotting
Physiological, excess ethylene
Tipburn
Physiological, Calcium deficiency and high temperature

Nematodes, Parasitic

False root knot
Nacobbus aberrans (Thorne) Thorne and Allen
Lesion
Pratylenchus spp.
Needle
Longidorus spp.
Root-knot
Meloidogyne spp.
Spiral
Rotylenchus robustus (de Man) Filipjev
Stunt
Melinius brevidens (Allen) Siddiqi
Tylenchorhynchus spp.

Parasitic Higher Plant

Broomrape
Orobanche ramosa L.

Virus and Viruslike Diseases

(Also mycoplasmalike organism [MLO])

Big vein
Lettuce big vein virus
Calico
Alfalfa mosaic virus
Tobacco ringspot virus
Internal rib necrosis
Lettuce mosaic virus
Mosaic
Lettuce mosaic virus
Biden mosaic virus
Cucumber mosaic virus
Turnip mosaic virus
Necrotic yellows
Lettuce necrotic yellows virus
Ring necrosis
Presumed viral agent (not identified)
Rusty brown discoloration
Accentuated by lettuce mosaic virus
Speckles
Lettuce speckles virus and beet western yellows virus
Stunt
Beet yellow stunt virus
Virescence
Aster yellows MLO
Beet leafhopper-transmitted virescence MLO
Wilt
Broadbean wilt virus
Tomato spotted wilt virus
Yellow spot
Tobacco rattlevirus
Yellows
Beet western yellows virus
Other
Sow thistle yellow vein virus