This disease is caused by Rhizoctonia solani. This fungus is one of the most common soilborne plant pathogens in agricultural soils of Arizona and 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.
Symptoms: There are no obvious above ground symptoms of bottom rot under the arid environmental conditions prevalent in Arizona. 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 (Fig. 12). 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.
Figure 12. Head lettuce with bottom rot infection.
Disease Cycle: Rhizoctonia solani can survive indefinitely in soil because it is an active colonizer of soil organic matter. Rhizoctonia 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 optimum temperature for growth of the pathogen is, with minimum and maximum temperatures for mycelial growth of 41 and 96° F.
Management and control: Growers should avoid planting lettuce immediately after other crops known to be susceptible to Rhizoctonia, such as alfalfa. Fungicides, such as iprodione (Rovral) and vinclozolin (Ronilan), 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 minorand S. sclerotiorum. Leaf drop was first described in Arizona in 1925 and 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.
Symptoms: The name 'leaf drop' best describes the prominent and most obvious symptom of this disease (Fig. 13). The pathogen usually invades the main stem or upper root, causing a soft, dark, watery decay. The destruction of stem tissue causes a rapid wilt, collapse and death of infected plants. In Arizona, this symptom is likely to be observed as 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.
Figure 13. Lettuce affected by leaf drop.
Disease Cycle: 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. minorand 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 2 to 3 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° F, with an optimum temperature range between 60 and 70° F. Sclerotia may survive up to 10 years in dry soil, whereas they decay in moist soil and may rot in less than a year.
Management and Control: 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 that 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, such as iprodione (Rovral) and vinclozolin (Rovral),
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.
Symptoms: 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 (Fig. 14). 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.
Figure 14. The fungal mycelium of downy mildew is evident only on the underside of the leaf.
Disease Cycle: 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° F, respectively. The pathogen can overseason in crop debris and soil as thick-walled oospores.
Management and Control: 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. Some fungicides that are effective for control of downy mildew on lettuce include maneb, fosetyl-Al (Aliette), and metalaxyl (Ridomil). Several experimental fungicides have shown promise in controlling downy mildew and may be available in the future. To achieve maximum benefits from fungicides, they must be applied when environmental conditions favor disease development and before the appearance of disease symptoms. Insensitivity to the systemic fungicide metalaxyl has been documented for some time. Alternating use of different chemistries as well as utilization of other resistance management strategies are strongly encouraged to impede the development of resistance to other fungicides by Bremia lactucae.
Powdery mildew, caused by the obligate parasitic fungus, Erysiphe cichoracearum, may occur on lettuce from January through April in desert production areas. This disease primarily affects plants approaching maturity, occurs in the absence of free water on leaf surfaces and is often confused with downy mildew.
Symptoms: Symptoms begin as small tufts of fungal growth on upper or lower leaf surfaces. As the disease progresses, the leaf area covered by the fungus increases, eventually coating the entire leaf with a powdery or dusty appearing growth (Fig. 15). Masses of spores produced in chains are found with the fungal growth on leaf surfaces.
Figure 15. Powdery mildew infects both upper and lower leaf surfaces.
Powdery and downy mildew can be differentiated by the following characteristics: 1) the powdery mildew pathogen produces spores in long chains on a single stalk on both sides of a lettuce leaf, whereas the downy mildew pathogen produces spores singly on branched stalks only on the underside of infected leaves; 2) powdery mildew can spread over the entire leaf surface and appears as a white dusty growth, whereas downy mildew lesions are delineated by leaf veins; 3) powdery mildew can occur in the absence of free water on the leaf surface, whereas downy mildew only occurs during wet humid weather when free water is present on leaf surfaces.mildew.
Disease Cycle: Conidia (spores) from infected plant tissue can be blown long distances to infect healthy leaves. Temperatures between 65-77° F are most favorable for germination of conida and subsequent infection of leaves. Relative humidity of 85% or above is required for infection, growth of mycelium and sporulation. From 4-10 days are required from initial infection to production of a new crop of conidia. Low light intensity tends to favor development of powdery mildew.
Management and Control: If available, planting of resistant varieties is advised for lettuce that will mature in late winter to early spring in desert production areas. Application of sulfur to leaf surfaces before the onset of disease when environmental conditions are favorable can effectively inhibit disease development. New chemistries have been tested and found to effectively control the development of powdery mildew on lettuce. Some of these materials could be available in the future as chemical disease management tools.
Big vein is caused by a virus-like agent that is carried or vectored by the soil-borne, root inhabiting fungus Olpidium brassicae. The disease was first reported in the Imperial Valley of California in 1934.
Figure 16. Lettuce infected with the Big Vein virus.
Symptoms: Big vein was named for its most prominent symptom, a pronounced clearing of the chlorophyll adjacent to major veins giving the appearance of enlarged veins (Fig. 16). Leaf veins are prominent when held up to bright light. Leaves of infected plants are more upright and have a more ruffled puckered appearance compared to healthy plants. In the field, conspicuous symptoms normally occur approximately one month after seeding and only when air temperatures during the day are below 65° F. Constant temperatures above approximately 70° F prevent big vein symptom development. Infected plants mature more slowly, may fail to form a head or produce small inferior heads.
Disease Cycle: Olpidium brassicae serves two important functions in the big vein disease. Zoospores of this fungus are produced under saturated soil conditions and transport the pathogen internally and inoculate it into lettuce root cells. Resting spores of Olpidium carry the pathogen internally and allow it to survive in soil from crop to crop for a minimum of eight years. Olpidium has a wide host range, including wild species of lettuce, celery, radish, onion and broccoli. In Aguila, Arizona the incidence of big vein was over 60% in a field first planted to lettuce, providing compelling evidence that the vector and the virus-like agent were present in the soil prior to planting. Disease incidence is higher in heavy textured poorly drained soils where zoospore production is favored by saturated soil conditions.
Management and Control: An effective management strategy for big vein is to plant varieties of lettuce that are tolerant to the disease. Fields with a history of big vein should not be planted with a lettuce crop that will mature when temperatures
favor infection and symptom expression. No cost-effective chemical soil treatments are available.