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Disease management in Arizona lettuce

Western vegetable production will soon shift from the California coastal production areas to the Arizona desert, where there are about 75,000 acres of leaf and head lettuce produced each year. The majority is in the lower Colorado River and Gila River valleys of Yuma County where elevation is below 100 feet.

Winter vegetable production in the desert has grown dramatically over the past two decades, and with it has come a variety of new challenges, including disease control.

The following information on controlling diseases in lettuce was developed by University of Arizona specialists to help growers in controlling yield-robbing diseases.

This disease is caused by Rhizoctonia solani, a fungus that is one of the most common soil-borne plant pathogens in agricultural soils of Arizona. It survives by colonizing soil organic matter.

Bottom rot is most widespread on early season lettuce that matures from mid- to late-autumn, with disease developing on plants that have headed and are nearly mature. In Arizona, bottom rot can be found when environmental conditions favor disease development and bottom leaves of lettuce plants touch the soil.

There are no obvious aboveground symptoms of bottom rot under the arid environmental conditions prevalent in the state. Mature lettuce plants appear normal until they are cut at harvest. Infected plants display sunken, reddish-brown lesions of varying depths and sizes on leaf petioles and midribs that touch the soil.

White to brownish mycelium grows over these lesions. The fungus can move upward from leaf to leaf until the entire head is colonized. Affected leaves are removed in the field prior to packing. Plants with extensive deep lesions are usually left in the field. Leaf tissue infected with Rhizoctonia solani may also be invaded by soft rot bacteria, resulting in a slimy decay that may cause additional losses of infected lettuce heads.

Rhizoctonia solani can survive indefinitely in soil because it is an active colonizer of soil organic matter. It is spread by any means that moves soil from one place to another. The pathogen can be carried long distances on infected plant parts, which can bear sclerotia of the fungus. The sclerotia can germinate in damp soil and enter plants through wounds or through stomata on leaves touching the soil. The minimum and maximum temperatures for mycelial growth are 41 and 96 degrees.

Growers should avoid planting lettuce immediately after other crops known to be susceptible to Rhizoctonia, such as alfalfa. Fungicides can be effective management tools for bottom rot on lettuce. Fungicide application should be initiated early, just after thinning, to achieve maximum suppression of pathogen activity.

Leaf drop of lettuce in Arizona is caused by two pathogenic fungi, Sclerotinia minor and S. sclerotiorum. Leaf drop probably occurs in all lettuce-production areas of the world when cool, moist conditions exist. The fungi can persist in soil for long periods of time in the form of overseasoning sclerotia. S. sclerotiorum, and to a lesser extent S. minor, can cause disease in a wide variety of different plants, including many vegetable crop plants.

The name ‘leaf drop’ best describes the prominent and most obvious symptom of this disease. The pathogen usually invades the main stem or upper root, causing a soft, dark, watery decay. The destruction of stem tissue causes the rapid wilt, collapse and death of infected plants. In Arizona, this symptom is likely to be observed when the plants gain enough size to cover the majority of the plant bed. Dense masses of white fungal growth appear on the surface of rotted tissue near the soil surface.

Hard, black, irregularly shaped fungal structures called sclerotia are produced on and in the rotted host tissue. The pathogen can be identified by the size of sclerotia produced on decayed lettuce tissue. S. minor produces small sclerotia 1/16 to 1/8 inch in diameter, whereas the larger sclerotia produced by S. sclerotiorum are usually from 1/4 to 3/8 inch in diameter and sometimes larger.

These pathogens carry over from season to season as active mycelium in dead plant tissue and as sclerotia in soil. Mycelium emerging from germinating sclerotia of S. minor and S. sclerotiorum can infect lettuce plants directly through senescent lower leaves and through root tissue near the soil surface. Sclerotia of S. sclerotiorum can also germinate in shaded areas on the soil surface during wet weather, by producing small flesh-colored flat or cup-shaped structures called apothecia, which are borne on short stalks approximately 1/4 inch in length.

Apothecia forcibly release ascospores into the air for a period of two to three weeks. These ascospores are carried by air currents and deposited on healthy lettuce plants, which subsequently become infected. Sclerotia germinate in moist soil during cool weather. Sclerotinia can cause infection from 32 to 82 degrees, with an optimum temperature range between 60 and 70 degrees. Sclerotia may survive up to 10 years in dry soil, whereas they decay in moist soil and may rot in less than a year.

There are several management strategies that can be implemented to minimize losses due to leaf drop caused by both species of Sclerotinia. (1) Avoid excessive irrigations, which tend to wet the top of lettuce beds. This is especially important when lettuce plants begin to cover a majority of the bed surface. Remember: wet soil stimulates sclerotial germination and plant infection. (2) Deep plowing tends to bury sclerotia, which promotes rotting of these fungal propagules and reduces their ability to germinate and cause infection. (3) Crop rotations with resistant crops such as corn and grasses should be used in problem fields. (4) Chemical management tools can provide effective disease control when applied promptly after thinning according to label recommendations.

Downy mildew, caused by the obligate parasitic fungus Bremia lactucae, usually can be found in lettuce fields each year in Arizona. However, both the incidence and severity of the disease are governed by the frequency and duration of cool moist conditions required for disease development. Free moisture on the leaf surface is essential for spore germination and infection, but not growth of the fungus within the leaf. A limiting factor for development of downy mildew in desert production areas is the rare occurrence of persistent cool weather, combined with high humidity and rainfall.

The first symptoms of downy mildew on lettuce are pale yellow regions on the upper side of older wrapper leaves, with a corresponding white fluffy growth, which contains the spores of the pathogen, on the lower leaf surface. The infected areas are limited by leaf veins. The spores (sporangia) of the fungus are produced singly on branched treelike structures. Affected tissue will eventually turn brown in color.

Sporangia from leaf spots are released into the air and blown long distances by prevailing winds to healthy leaf tissue. In the presence of free water on the leaf surface, the spore germinates and can penetrate and infect epidermal cells within three hours.

Following establishment of the fungus in leaf tissue, fruiting stalks emerge through stomates on the affected lower leaf surface, branch repeatedly, and produce an abundant crop of sporangia, which are released into the air to cause new infections. In Arizona, this cycle often is suppressed by dry weather that follows rainy periods. Minimum, optimum and maximum temperatures for infection are approximately 40, 50 to 72, and 86 degrees. The pathogen can overseason in crop debris and soil as thick-walled oospores.

Downy mildew can be successfully managed by planting varieties of lettuce with tolerance or resistance to the pathogen when available. Several different pathovars of Bremia exist, some of which can infect varieties that were formerly resistant. Timely application of fungicides, when environmental conditions are favorable, can effectively suppress disease development.

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