Foliage Wilts, Shows Dark Spots
Four-lined Plant Bug - The four-lined plant bug is a common pest of perennial plants. Adult insects are flat and thin, about 1/4 inch long. They get their name from the 4 black stripes that run the length of their greenish yellow backs. Young bugs are brightly colored red or orange with distinctive black spots behind their heads and eventually a yellow stripe on the sides of their wing pads. They feed for about 6 weeks, rapidly ducking out of sight if approached. As they suck juices from plant leaves, they inject a poison into them which causes the damaged area to darken. Sometimes several spots on a leaf will merge, the leaf will turn brown, and drop off. Use spray early in the morning when the bugs are least active. Spray pyrethrum laced with isopropyl alcohol directly on the bugs, once every 3 days. If the infestation is heavy, dust plant surfaces with sabadilla. The best way to control this pest is with thorough fall and spring cleanup of weeds and plant debris that harbor eggs over the winter. For more information see the file on Controlling Plant Bugs
Dark Spots On Leaves Show Yellow Edges
Bacterial Leaf Spot - A bacterial leaf spot disease is difficult to distinguish from a fungal one. It helps to be able to do this, because it is important to treat the correct disease. Similar to those infected by fungi, leaves infected by bacteria show small and blisterlike darkened spots which gradually enlarge and run together. Bacterial leaf spots rupture release a bacterial ooze. In serious infections, plants may be defoliated and collapse. While leaf spot diseases of all kinds are aggravated by overhead watering, fungal ones thrive in cool, moist environments, and bacterial ones rapidly develop in the heat. Spots caused by bacterial infection are sometimes ringed in yellow, signs that the toxin produced by the bacteria are moving out into surrounding tissues.
As there is no effective chemical control of bacterial plant diseases available, the only recourse is to remove and discard infected leaves as soon as spots appear. Dig out heavily infected plants along with the soil around their rootball and discard them in the trash to prevent the spread of the bacteria. Do not compost this material. Keep the garden free of plant debris. Disinfect garden tools with 70 % denatured alcohol. Control insects (such as aphids) that may transmit disease organisms. Increase air circulation by spacing plants more widely apart, and remove the lower 4 to 6 inches of foliage to avoid contact with wet soil.
Pale Patches On Leaf Undersides; Foliage Wilts
Downy Mildew - -Downy mildews are caused by fungi that grow and spread on moist leaf and stem surfaces. Nearby plants, weeds, or even seeds may harbor the infection. These fungi form pale areas on upper leaf surfaces and gray or white or purplish "downy" patches on undersides. Older leaves are usually affected first, often turning brown and dying. Mildew fungi are especially active in periods of cool wet nights and warm humid days, eventually causing most leaves to wilt and die.
Plant cranesbill on well-drained, fertile soil; water early on sunny days. Avoid crowding the plants. Minimize foliage dampness by watering just the soil. Dig out heavily infected plants and any adjacent soil and discard them in the trash. Do not compost them. Because fungal infection may remain in the soil, plant a different species in this area for a few years. Spray lightly infected plants with flowable sulfur or dust them with lime sulfur or Bordeaux mixture when downy mold first appears on leaf undersides. Several applications may be needed to achieve control. For more information see the file on Controlling Fungal Disease