Click image for larger view. Return to: MREC Home Page
Return to: MREC Research Index
A. R. Chase*
University of Florida, IFAS
Central Florida Research and Education Center - Apopka
CFREC - Apopka Research Report, RH-90-23
The following report summarizes research performed to evaluate the potential for use of fertilizer to reduce severity of Phytophthora stem rot of Aphelandra squarrosa 'Dania'. Symptoms of Phytophthora stem rot are wilting and collapsed stems due to rotting at the soil-line (1).
Materials and Methods
Plant production. Rooted cuttings of A. squarrosa 'Dania', obtained from commercial producers, were used in two tests performed between 9 January and 2 August 1990. Plants were grown in 6 inch pots using a steam-treated medium consisting of Canadian peat and pine bark (1:1, by volume). Light levels ranged from 2000 to 2600 ft-c. with temperatures between 65 and 95°F. Ten plants were used for each of seven rates of Osmocote 19-6-12 (applied as a top-dressing): 0.7, 2.9, 5.1, 7.3, 9.5, 11.7, and 13.9 g/6 inch pot. The recommended rate for aphelandras under these conditions is about 5 g (4). Plant height, number of leaves, quality and leachate soluble salts [using the pour-through method (5)] were recorded 8 weeks after fertilizer was applied. Plant quality was rated using the following scale: 1 (dead), 2 (poor, unsalable), 3 (marginal, salable), 4 (good, salable), and 5 (excellent, salable).
Inoculation. Plants were inoculated, two months after fertilization, with a mycelial slurry of Phytophthora parasitica grown on potato-dextrose agar medium and watered lightly into the potting medium.
Results
Leachate soluble salts increased as fertilizer rate increased and ranged from 300 to 6500 µmhos/cm. Increasing the Osmocote rate also affected height, plant quality, and number of leaves of aphelandra. Rates between 5.1 and 13.9 g Osmocote gave good growth (Fig. 1 - 3). Only plants receiving the lowest rate of fertilizer (0.7 g) showed substantially reduced growth.
The severity of stem rot caused by P. parasitica was rated on a scale from 1 (no disease) to 5 (dead) was highest for plants receiving 9.5 g or higher rates of Osmocote (Fig. 4). None of the under fertilized plants showed any signs of stem rot.
Summary
When fertilizer rate is to be used as a cultural control method several factors must be considered. Rates which reduce disease significantly must not result in plant damage, excessive costs (due to high rates), or significantly increased potential for ground water contamination. Fertilizer rates which result in the lowest severity of Phytophthora stem rot of Aphelandra corresponded to best plant growth (about 3 to 7 g Osmocote 19:6:12/6" pot). This range of rates is close to those previously recommended for production of Aphelandra. An additional consideration is that fertilizer rates used to decrease one disease should not increase the potential for other diseases. Previous research on Aphelandra and Dieffenbachia showed that increased fertilizer levels resulted in increased severity of Myrothecium leaf spot disease (2, 3). Fertilizer rates which reduce severity of Phytophthora stem rot of Aphelandra would probably reduce severity of Myrothecium leaf spot as well.
*Professor of Plant Pathology, Central Florida Research and Education Center Apopka, 2807 Binion Road, Apopka, FL 32703-8504.
Literature Cited
Figure 1. Effect of Osmocote 19-6-12 rate on height of Aphelandra squarrosa 'Dania'.
Figure 2. Effect of Osmocote 19-6-12 rate on number of leaves on Aphelandra squarrosa 'Dania'.
Figure 3. Effect of Osmocote 19-6-12 rate on plant quality of Aphelandra squarrosa 'Dania'. Plant quality was rated on a scale from 1 (dead) to 5 (excellent, salable).
Figure 4. Effect of Osmocote 19-6-12 rate on severity of stem rot disease on Aphelandra squarrosa 'Dania' caused by Phytophthora parasitica. Disease severity was rated from 1 (no disease) to 5 (dead).
Click image for larger view.
Click image for larger view.
Click image for larger view.