Edge burn is beginning to appear on geraniums and marigolds where growth medium pH is low (below 6.0). As pH decreases, iron and manganese become more soluble. Some plants are very efficient at taking up iron (Fe)/ manganese (Mn) in the tissue causing toxicity symptoms. See photos, "Iron Toxicity" Crops such as zonal and seed geraniums, marigolds, lisianthus and all types of impatiens are susceptible. This disorder is sometimes called "bronze speckle" due to the appearance of numerous small brown spots on the leaves.
A common problem often diagnosed this time of year with spring crops is excess soluble salts. Generally, this is a result of too much fertilizer in relation to the plants needs. Inadequate watering or leaching, or poor drainage can also result in high soluble salts.
Sometimes high soluble salts levels occur when root function is impaired by disease or physical damage. Always check the condition of the root system when trying to diagnosis a problem.
Two-spotted Spider Mites are developing, especially on hanging baskets, in hot dry areas of greenhouses. Don't forget to check hanging baskets of New Guinea impatiens, ivy geraniums, zonal geraniums, thunbergia, scalevola or mixed baskets with Ipomoea "Blackie" for two-spotted spider mites. Herbaceous perennials in hot, dry areas of a greenhouse are also susceptible. Look on the underside of the oldest leaves along the midveins, for the mites, their eggs and white empty eggshells. You may see faint stippling, or chlorotic growth that resembles a nitrogen deficiency.