Read about these observations and more from Floriculture Greenhouse Update.
- Downy mildew
- Pesticide Use
- Ammonium Toxicity
Wednesday, 18 May 2011 12:59
Downy mildew has been observed on basil. If you suspect your basil plants are suffering from a nutrient deficiency, you may want to inspect plants more carefully, for Downy Mildew. Symptoms are similar especially before visible spores develop. See photos: Basil Plant, Close-up on basil leaves,Undersides showing spores
Clean Plants: It is especially important during cloudy, rainy weather to keep plants in retail areas clean even during the busiest times. Removing dead and injured plants and spent flowers are essential at least twice a day even during the busy season. Botrytis and high ethylene concentrations from decaying plant tissue will cause premature loss of foliage and flowers.
Botrytis: Based upon past grower experiences, Decree with capsil is still probably your first choice in terms of plant safety. However, the label does state “Do not make more than two consecutive applications of this product. After the second application, use an alternative fungicide effective in controlling Botrytis for two applications before reapplying the active ingredient in this product.” Decree will perform better with a surfactant such as CapSil added (8 oz per 100 gallons or ½ teaspoon per gallon) to increase coverage and reduce visible residue.
Considering plant safety and residue as of primary concern, one possible alternative is the preventive biological fungicide CEASE.
Watering: During prolonged cloudy, rainy weather, let plants run on the dry side and spot water areas as needed to prevent over-watering. When some plants get spot watered and are irrigated more frequently than the general crop, their fertilizer program needs to be adjusted to account for the extra leaching they receive. They may need to be "spot fertilized" as well. To prevent foliar diseases likeBotryis, always water in the morning to reduce the length of time the leaves stay wet after irrigating. Avoid watering late in the day or when water will sit on leaf surfaces for long periods of time.
Pesticide Use: If pesticides are needed, read precautions for treatments. During cloudy weather, some pesticides such as horticulture oil may burn foliage if it remains wet on the foliage and does not dry quickly. See photo.
Ammonium Toxicity: Low temperatures (less than 60°F average daily temperature), water-saturated or low oxygen growing media, and low medium pH can cause ammonium to build up to toxic levels in the growing medium. Symptoms of ammonium toxicity include upward or downward curling of lower leaves depending on plant species; and yellowing between the veins of older leaves which can progress to necrosis.To avoid ammonium toxicity it is recommended to use fertilizer with 40 percent or less ammoniacal nitrogen when growing conditions are cool and wet. When cool, cloudy weather occurs a common strategy used by growers is to alternate to a 15-0-15 “Dark Weather Formula.” This eliminates the extra ammonium input temporarily, but then allows growers to return to their normal formula when growing conditions improve.
Reference: Nitrogen: Not All Forms are Equal by Cornell University, Fact Sheet with ammonium toxicity photos.
Photo: “Flagging” on geraniums caused by high soluble salts and ammonium.From Floriculture Greenhouse Update by Extension Specialists Tina Smith (UMass), Leanne Pundt, (UConn) & Paul Lopes (UMass).