Effect of Osmocote rate on growth of Impatiens Walleranaand severity of Pseudomonas leaf spot

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A. R. Chase*

University of Florida, IFAS
Central Florida Research and Education Center - Apopka
CFREC - Apopka Research Report, RH-90-19

Impatiens Wallerana (common impatiens) are popular bedding plants throughout the United States. Impatiens are not cold tolerant but can be grown year round in Florida when shade and water conditions are appropriate. During the past two years Pseudomonas leaf spot has been found and described on impatiens. The cause of this disease has been identified as Pseudomonas syringae. The disease may be present throughout the year although symptoms may only develop when temperatures reach an optimum. New Guinea impatiens have also been found infected with P. syringae. The following paper describes research performed to evaluate the affect of fertilizer level on growth of impatiens and development of Pseudomonas leaf spot.

Materials and Methods

Plugs of Impatiens Wallerana (common impatiens) were obtained from commercial producers and planted in 5-inch round pots containing a steam-treated medium consisting of Canadian peat and pine bark (1:1, by volume). Plants were irrigated from overhead at least three times/week through the trial period. Light levels ranged from 2600 to 3000 ft-c. with temperatures between 65° and 95°F. Ten pots were used for each of six rates of Osmocote 19-6-12 (a 3-4 month release term formulation applied as a top-dressing): 3, 6, 9, 12, 15, and 18 g/5 inch pot. Plant height and leachate soluble salts (using the pour-through method) were recorded every 2 weeks after fertilizer was applied. Plant quality was rated after 4 weeks using the following scale: 1 (dead), 2 (poor, unsalable), 3 (marginal, salable), 4 (good, salable), and 5 (excellent, salable). Plants were inoculated with a bacterial suspension of P. syringae after top quality was evaluated. The number of lesions per plant was recorded 5 - 7 days after inoculation. Three tests were performed between 15 May and 31 July 1990. Results and Discussion

Results from the three tests were similar and only those from test 2 are presented. Leachate soluble salts increased as fertilizer rate increased and ranged from 375 (for plants without fertilizer) to 3200 µmhos/cm after 2 weeks and 100 to 750 µmhos/cm after 4 weeks. Increasing the Osmocote rate increased final height and plant quality with best plants produced with rates between 9 and 18 g Osmocote 19:6:12 (Table 1). Number of lesions caused by Pseudomonas syringae increased as amount of fertilizer increased (Table 1).

Summary

The rapid decrease in leachate soluble salts indicates problems many growers and homeowners face in supplying an adequate amount of fertilizer when plants are produced under overhead irrigation during hot weather. Since all rates above the 3 g rate gave equivalent responses, the lowest rate (9 g should be used) and possibly applied more frequently than every 90 days to avoid limiting the growth of the crop due to lack of fertilizer. Use of higher rates is not recommended since the excess fertilizer was apparently leached from pots and was lost. If Pseudomonas leaf spot is a problem use of the 9 g rate may reduce development of this disease on Impatiens.

*Professor of Plant Pathology, Central Florida Research and Education Center Apopka, 2807 Binion Road, Apopka, FL 32703-8504.

Fertilizer rate significantly affected each parameter recorded.