Greenhouse - Geranium Pointers and Troubleshooting

The following are some pointers and troubleshooting hints for geranium production from the University of Maryland.

Sudden pH Drops:

Your geraniums may be humming along but make sure you are maintaining a correct pH level. If you let the pH fall off the plants will go downhill rapidly. Check your pH level at least weekly to detect any downward turns. There is a condition called sudden pH decline (SPD) where geranium crops growing at the optimum pH rapidly (1- 2 weeks) have the substrate pH shift downward 1 to 2 units. Taylor et al., 2008, reports that this occurs when phosphorus is deficient in the substrate and high temperatures cause stress of the geranium crops.

Proper pH:

The optimal pH varies by the type of geranium and root medium used. For zonal geraniums, the range for a soilless root medium is 5.8 to 6.2 and for a soil-based medium is 6.0 to 6.5. The optimal range is up to 0.3 units lower for ivy and regal geraniums (5.5 - 6.2). If modifications are required, the pH can be lowered with an acid-based fertilizer or acid injection. The pH can be increased with dolomitic limestone or hydrated lime.

Electrical Conductivity (EC):

The optimal EC range is 1.5 to 2.5 mS/cm for zonal and regal geraniums. Slightly lower levels are required by ivy geraniums, usually 1.0 – 2.0 mS/cm. Aphids on Geraniums: Monitor ivy geraniums for melon aphids and green peach aphids. We are seeing populations increasing in some greenhouses this week.
Control: Imidacloprid (Marathon), dinotefuran (Safari), Acetamiprid (TriStar), Endeavor, Aria, insecticidal soaps.

Diseases to watch out for on Geraniums:

One disease to watch out for on geraniums is bacterial leaf spot. This disease is caused by the bacterium Xanthomonas pelargonii and is especially prevalent in warm, wet weather where plants are grown in crowded conditions. Disease symptoms include small (pinhead size), circular or irregular, brown, sunken spots on older or lower leaves. Large numbers of spots will occur on a single leaf, these will coalesce and can kill a large portion of the leaf which will then drop off. As the disease moves through the plant, the lower leaves wilt and yellow. In severe cases, the stem will possess black stem cankers killing the upper portion of the stem. Leaves infected with bacterial leaf spot should be removed as soon as it is noticed. Severely infected plants should be removed.

Monitor for Botrytis leaf spot or blossom blight. It is caused by Botrytis cinerea. Botrytis is favored under cool, moist conditions or where plants are watered frequently. Leaves develop zonate, brown leaf lesions which develop a grayish brown mass of fungal spores. The lower leaves will yellow and rot. Flowers may also become infected. They show discolored petals which wilt and fall. Remove affected leaves and flowers. Maintain good air circulation in the greenhouse.

Reprinted from the April 3, 2009 edition of the Greenhouse TPM/IPM Bi-Weekly Report from the University of Maryland Cooperative Extension, Central Maryland Research and Education Center http://www.ipmnet.umd.edu/09Apr03G.pdf