Disease Management

Lettuce Septoria Blight

Leaf Spots of Lettuce

Lettuce is susceptible to various fungal (Septoria lactucae, Cercospora longissima, Microdochium panattonianum) and bacterial (Xanthomonas campestris pv. vitians) leaf spots. Because the leaf is the harvested product, leaf spots can cause considerable damage under the right environmental conditions.

Septoria lactucae occurs worldwide and becomes important under prolonged high humidity and frequent rainfall. Symptoms generally start on oldest leaves and consist of small, irregular chlorotic spots. Lesions enlarge, turn brown, dry out and may fall out giving the leaves a tattered appearance. Numerous fruiting bodies (pycnidia) develop with leaf spots. The pathogen survives in lettuce seed, crop debris, and on wild lettuce hosts.

Cercospora leaf spot is seldom destructive except in the tropics and subtropics. Symptoms of small brown spots with chlorotic haloes occur on the lower leaves first. The disease spreads upward and lesions become sunken and necrotic. The pathogen survives in infected crop debris and wild lettuce hosts. Spread of the disease is dependent on long periods of leaf wetness.

Microdochium panattonianum is also known as anthracnose, shot-hole, and rust. The pathogen is distributed worldwide and can cause considerable losses when conditions are cool and moist. Symptoms first appear as tan, water-soaked lesions on the midrib of outermost leaves. Lesions expand, become straw colored, and the centers fall out. Severe infections affect head formation and may be invaded by secondary rotting organisms. The pathogen survives in infected crop debris or as microsclerotia in soil. Spread is primarily by splashing water.

Bacterial leaf spot can cause serious losses in the United Sates. Early symptoms are angular, water-soaked spots which quickly turn black. Lesions may coalesce and result in leaf collapse. Like the fungal leaf spots, X. campestris pv. vitians is dependent on cool moist conditions to cause disease .The bacteria may be seed-borne (infrequently), survive in the soil for several months, or persist on wild lettuce hosts.

Management:

• Start with certified, disease-free seed.
• Examine transplants and remove infected plants.
• Avoid overhead irrigation if possible and minimize periods of leaf wetness..
• Plow plant debris under after harvest to hasten decomposition.
• Rotate with nonhost crops.
• Control wild lettuce weeds.
• Avoid working in fields when plants are wet.