By L.J. du Toit and C.M. Ocamb
Spinach (Spinacia oleracea) - Leaf Spot
Cause A fungus, Stemphylium botryosum f. sp. spinacia, first found in California in 1997, reported in Oregon, and found in spinach seed crops in Washington in 2000. Although there are many crops with leaf spot caused by S. botryosum, the isolates pathogenic on spinach do not appear to be pathogenic on other crops, and vice versa. The fungus can grow under a wide range of temperatures, but does most damage at 65°F to 74°F when relative humidity is above 80%. Spores are disseminated by air, rain or irrigation splash, or on equipment. The fungus can also be seedborne, not only on the pericarp but also in the embryo. After harvest of spinach seed crops, the sexual stage of the fungus, Pleospora herbarum, forms black, pinhead-size fruiting bodies (pseudothecia) on spinach crop residues that remain on the soil surface (especially 'woody' stems and branches). P. herbarum does not form fruiting bodies on buried residues. The fruiting bodies mature over the winter and forcibly discharge spores into the air in the spring when temperatures increase.
Symptoms At first, leaf spots are pinpoint in size, circular, and sunken, but expand to become gray-green and up to 0.25-inch in diameter with an oval or round shape. Leaf spots expand and turn tan in color as disease progresses; finally, coalescing leaf spots turn brown as infected leaf tissue dries out and turns papery. Disease can be confused with spray injury or leaf spot caused by Cladosporium variabile, although lesions caused by the latter tend to be smaller, remain circular, and develop a narrow darker margin as they mature. Symptoms develop most rapidly on older leaves, and severe disease pressure can cause defoliation.
- After harvest of spinach seed crops, incorporate residues into the soil to prevent the sexual stage of the fungus from forming fruiting bodies on residues that remain on the soil surface. Disc volunteer plants and residues into the soil before the winter so that inoculum does not mature on the residues over the winter.
- Hot water treatment of spinach seed at 104°F to 122°F for 10 to 20 min. can be partially effective. Seed must be dried well after treatment to avoid reduction in shelf-life.
Chemical control Fungicides used preventively help minimize the onset of infection and disease development. Spinach pollen makes S. botryosum much more aggressive as a pathogen of spinach. Therefore, protective fungicide applications are recommended just prior to initiation of pollen shed in spinach seed crops.
- Bravo Weather Stik (Group M5) at 3 pints/A on 7- to 14-day intervals; begin applications just prior to pollen shed. For spinach seed crops in Oregon only (SLN OR-200014) and Washington only (SLN WA-190001). 48-hr reentry.
- Dithane DF Rainshield (Group M3) at 2 lb/A on 7- to 10-day intervals can be used spinach seed crops only. Oregon and Washington only (SLN OR-020030, SLN WA-020028). 24-hr reentry.
- LifeGard WG (Group P6) at 1 to 4.5 oz/A on 7- to 14-day intervals for activating plant resistance. Refer to label for appropriate rate per application volume. Preharvest interval is 0 days. 4-hr reentry. O
References du Toit, L.J., Derie, M.L., and Brissey, L.M. 2007. Evaluation of fungicides for control of seedborne Stemphylium botryosum on spinach, 2006. Plant Disease Management Reports 1:ST003.
du Toit, L.J., Derie, M.L., and Hernandez-Perez, P. 2004. Evaluation of fungicides for control of leaf spot in spinach seed crops, 2003. Fungicide & Nematicide Tests 59:V115
du Toit, L.J. and Hernandez-Perez, P. 2005. Efficacy of hot water and chlorine for eradication of Cladosporium variabile, Stemphylium botryosum, and Verticillium dahliae from spinach seed. Plant Disease 89:1305-1312.