Magnesium (Mg) Deficiency of Tomato Plants
The most common symptom of magnesium deficiency is interveinal chlorosis (yellowing between veins) on the lower leaves, often occurring also on the older leaves. Magnesium is a mobile element; if it is limited in the plant, it will be translocated from the old mature leaves to the new growth (leaves). The initial symptoms of magnesium deficiency begin as faint interveinal chlorosis and as deficiency increases it becomes more pronounced. With advanced deficiency, necrotic (brown) spotting and dark purplish black spotting may develop. This may be confused with low substrate pH induced iron and/or manganese toxicity which mimics advanced magnesium deficiency.
To correct magnesium deficiency, apply Epsom salt (magnesium sulfate) at the rate of 2 pounds per 100 gallons of water. If fertilizers that do not contain magnesium are used, apply Epsom salts at the rate of 1 pound per 100 gallons of water monthly to avoid magnesium deficiency development.
Botrytis Blossom Blight on Petunia
Blossoms of plants such as petunias are very susceptible to Botrytis blight during stretches of cool, cloudy weather. The pathogen causes tan or off-color spots on fully opened flowers. Flower buds may be completely blighted before opening, and the fuzzy gray sporulation is typically visible.
The best way to prevent this disease is through good cultural practices. Remove spent flowers and dead leaves. Avoid overwatering and keep the plant canopy as dry as possible. Reduce relative humidity in the greenhouse by spacing plants properly and increasing air circulation.
Fungicides will not save blighted blossoms but may help protect new blossoms from becoming infected. Select fungicides from FRAC groups 3, 7, 9, 12, 17, and/or 19 and rotate among these. Avoid thiophanate-methyl, boscalid, and iprodione as resistant strains of Botrytis are believed to be widespread.
For more information on fungicides available to control Botrytis please consult the New York and New England Floriculture Guide.
Geoffrey Njue, Extension Specialist, UMass Extension Greenhouse Crops and Floriculture Program and Angela Madeiras, Plant Diagnostic Lab, UMass Extension