Scouting Notes - November 2007

Be on the lookout for control-resistant whitefly. This summer Society of American Florists advised growers to start scouting early for whiteflies to avoid serious problems later in the season. Growers were told to pay particular attention to their control programs to be sure they were effective.

Resistant whiteflies and/or increased numbers of whiteflies had been reported by growers in several parts of the United States. Q-biotype whitefly is resistant to many commonly used chemicals.

Growers who notice apparent resistance in whitefly populations should send samples to diagnostic labs for biotype identification. Whitefly samples can be sent to several labs with guarantee of anonymity and no regulatory action will follow. Knowing which biotype a grower is dealing with can help ensure application of the appropriate chemicals and avoid further development of more resistant whiteflies.

For more: Lance Osborne, University of Florida, lsosborn@ufl.edu; http://mrec.ifas.ufl.edu/LSO/bemisia/bemisia.htm.

Leaf spot can develop on susceptible garden mums. Some varieties of garden mums may be more susceptible to bacterial leaf spot during warm, wet weather. The disease, which is caused by Pseudomonas cichorii, tends to be more of a problem during periods of heavy rains or when overhead irrigation is used.

Disease symptoms include large black spots concentrated at the base of the plant. The spots often begin at the leaf margin but may also occur randomly. The spots are soft when tissue is wet and sunken and brittle when leaves dry.

The bacteria can move through the leaf petiole into the stem, resulting in a canker. The sepals of infected flower buds turn brown to black and several inches of pedicel can be killed.

University of Massachusetts extension floriculture specialist Tina Smith said key management strategies include starting with pathogen-free seed and cuttings, using resistant varieties, practicing good sanitation, avoiding overhead irrigation and avoiding handle plants when they are wet. Once plants become infected with bacteria, it is best to rogue infected plants.

For more: Tina Smith, University of Massachusetts, (413) 545-5306; www.negreenhouseupdate.info.

Soil test helps avoid misdiagnosis of plant problems. When University of Maryland cooperative extension agent Brian Clark submitted stunted and extremely chlorotic chrysanthemums for testing, the initial diagnosis was some type of nutrition problem based on the symptoms. However, once a soil test was performed the roots were found to be infected with Pythium.

The roots were discolored and water-soaked and not taking up nutrients. The outer cortex of the roots was being sloughed off, exposing the vascular tissue. Wet growing medium and high salt levels promote Pythium infection.

Suggested chemical controls include etridiozole (Truban), etridiozole + thiophanate-methyl (Banrot) and mefenoxam (Subdue Maxx).

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For more: Brian Clark, (301) 868-8780; bpclark@umd.edu.