Cause A physiological response by tomato plants that associated with low light conditions, high humidity, or possibly air pollutants. Low transpiration rates along with a rise in water absorption increase cell pressure, erupting epidermal cells so that inner cells enlarge and protrude. That causes the inner cells to potential die and discolor (yellow, brown, or rust), giving the impression that a parasitic organism is involved. The severity of symptoms depends on the variety of tomato.
CauseAlternaria brassicae and Alternaria alternata can infect leaves, petioles, stems, flower parts, and seed pods of sugar beet. Alternaria brassicae is a common pathogen on crucifer crops and weeds. Alternaria alternata can be a greater problem in plants with mechanical injuries or growing under water or nutrient deficiencies.
Cause Cucurbitacins, which are a group of plant-produced, bitter chemical compounds that occur naturally in this plant family. These compounds occur in a subset of the species in cucurbit plant family and are usually found at highest concentrations in the roots of plants. Over the years that humans have been developing cucurbit varieties, cucumbers, melons, squash and pumpkins lines have been selected to be free of high levels of these extremely bitter compounds.
Needle cast diseases can easily be confused with one another. Over a dozen different fungi have been associate with needle problems by the OSU Plant Clinic.
OSU Plant Clinic Image, 2014.
Cause Species of Pythium, including P. ultimum (syn. Globisporangium ultimum). These are fungus-like microorganisms that survive in organic matter in soil.
Cause The fungus, Neospermospora avenae (syn. Spermospora avenae), which infects oats and the weedy winter wild oat (Avena sterilis). It was first reported on oats in western Oregon and then Washington in the 1930s. It has occurred on the east side of Washington as well as in Idaho. It is rarely a significant problem in the Pacific Northwest but is commonly a problem on oat crops in Australia and can occur in the UK as well as other regions where oats are produced.
Cause The OSU Plant Clinic has identified 4 different leaf spotting fungi from past samples of mountain ash including Cercospora, Cladosporium (leaf scorch), Gloeosporium, and Septoria spp. not including rusts.
