Septoria Leaf Spot
(Text from T. A. Zitter, Department of Plant Pathology, Cornell University ; Fact Sheet Page: 735.80 Date:12-1987.) Septoria leaf spot of tomato caused by the fungus Septoria lycopersici occurs on tomatoes worldwide. The fungus infects only solanaceous plants, of which tomato is the most important. Tomatoes may often be infected with leaf spot and early blight (Altemaria solani) simultaneously, but the two diseases can be distinguished readily and the control measures are similar.
Septoria leaf spot can occur at any stage of plant development. Symptoms may appear on young greenhouse seedlings ready for transplanting or be first observed on the lower, older leaves and stems when fruits are setting. The timing of symptom appearance can be correlated with the sources of inoculum and environmental factors and will be discussed later. Small, water-soaked circular spots 1 .6 to 3.2 mm in diameter first appear on the undersides of older leaves. The centers of the spots are gray or tan and spots have a dark brown margin. As the spots mature, they enlarge to about 6,4 mm in diameter and may coalesce. In the center of the spots are many dark brown, pimple-like structures called pycnidia-fruiting bodies of the fungus. The structures are large enough to be seen with the unaided eye or with the aid of a hand lens. Pycinidia are absent from early blight lesions and from lesions produced by the gray leaf fungus, Stemphylium solani, which is common in areas with consistent warm and humid conditions. Septoria leaf spot also lacks the target-like lesions so typical for Altemaria blight. Spots may also appear on stems, calyxes, and blossoms, but rarely on fruit. Heavily infected leaves will turn yellow, dry up, and drop off. This defoliation will result in sunscalding of the fruit.
Under wet conditions, numerous spores (conida) are produced in the pycnidia and are exuded when the fruiting structures are mature. The temperature range for sporulation varies from 15° to 27°C with 25°C being optimal. Spores may be spread by windblown water, splashing rain, hands and clothing of pickers, insects such as beetles, and cultivation equipment. Following spread, spores may germinate within 48 hr under moist conditions and favorable temperatures. Leaf spots can appear within 5 days, pycnidia appear within 7-10 days, and spore production is repeated within 10-13 days. Because free moisture is necessary for spore infection through stomates, long-lasting dew and rainy days (100 percent RH for 48 hr accumulated over several days) favor disease development. Although the fungus is not a soil inhabitant, it can persist from one season to the next on debris of diseased plants incorporated in the soil (most commonly in the field but occasionally in the greenhouse). The pathogen can also overwinter on solanaceous weeds. Susceptible weeds include: jimsonweed (Datura stramonium), an annual reproducing by seed horsenettle (Solanum carolinense), a perennial reproducing by seed and rhizomes; smooth groundcherry (Physalis subglabrata), a perennial reproducing by rhizomes and seed; and black nightshade (Solanum nigrum), an annual reproducing by seed. Crops infected are potato and eggplant in addition to tomato. Tomato seed has been shown to carry spores and produce infected seedlings, but whether the pathogen is truly seedborne is unknown.