EFFECT OF NITROGEN AND POTASSIUM NUTRITION ON THE PHYTOPHTHORA LEAF SPOT OF PHILODENDRON

Transcription

1 HARKNESS AND REYNOLDS: PHYTOPHTHORA PHILODENDRON 47 EFFECT OF NITROGEN AND POTASSIUM NUTRITION ON THE PHYTOPHTHORA LEAF SPOT OF PHILODENDRON OXYCARDIUM Roy W. Harkness and J. E. Reynolds1 Sub-Tropical Experiment Station Homestead Introduction The tropical foliage plant industry in Florida is a million dollar industry according to Nicholls et ah (3). Philodendron oxycardium Schott is by far the largest single item in the trade, amounting to $3,400,000 according to the same report. Although this species is diminishing in importance in South Florida, almost every grower still produces some plants. During the rainy summer months, heavy losses are caused by a Phytophthora leafspot. The causative organism closely resembles Phytophthora nicotianiae var. parasitica (Dastur.), Waterhouse (Syn. P. parasitica Dastur.), which also causes leaf spots on several other philodendrons, Pothos Scindapsus aureus Engl. and buckeye rot of tomatoes (1). The fungus is believed to be spread by zoospores which mature in water in and around the Florida Agricultural Experiment Stations Journal Series No l Formerly at Sub-Tropical Experiment Station. Now at Grays Harbor College, Aberdeen, Washington. beds. Raindrops splashing in the water serve as an efficient means of zoospore dispersal. The lesions are dark brown, watersoaked spots that rapidly increase in size until the whole leaf is affected and falls to the ground. The fungus does not seem to invade the stem tissue. Chemical controls have been developed by Mc- Fadden (2) but are expensive and residues on the foliage detract from the appearance of the plants. Cultural practices that would reduce the severity of the disease are desirable, and this re port describes experiments toward this end. Materials and Methods The plants were grown in new sterile peat moss in 9 x 13 x 3 inch metal flats. Eight oneeye cuttings of P. oxycardium were put in each flat and watered until rooted. After the plants were rooted, the water was flushed out of the flats with an excess of nutrient solution and about 0 ml of nutrient solution was added to each flat once or twice a week thereafter. A preliminary experiment completed in 193 indicated that high nitrogen decreased disease severity, potassium probably had some effect and phosphorus seemed to be of less importance. Table 1. Solution analyseso PFM at time of inoculation. Treatment Ammonia Applied Nitrogen In Soil Phosphorus Applied In Soil Potassium Applied In Soil It ft

2 47 FLORIDA STATE HORTICULTURAL SOCIETY, 194 These results led to the choice of treatments listed in Table 2 which were started on July 31, 193. Each treatment was replicated four times. Ammonium nitrate was the source of nitro gen, monobasic sodium phosphate was the source of phosphorus, and a 1:1 ratio of potassium sul phate : potassium chloride was the source of potassium. The plants were allowed to grow until 3 or 4 new leaves were evident. The new growth was harvested and fresh weight determinations were made. This procedure was used to minimize the effect of prior nutrition of the cuttings and assess the uniformity of growth. The plants were allowed to grow again until 3 or 4 new leaves were evident. At this time, in February 194, the soil solutions were analyzed about three days after applying nutrient solution with the results shown in Table 1. (Nitrate nitro gen was not determined, so this table lists only half the total nitrogen). Since the compositions of the soil solution were Table 2. Effect of treatment on disease incidence and growth. Treatment, ppm Spots per No. N P K loog leaves February June Wt. of leaves Leaf Analysis in % Dry Weight Spots per. in grams Nitrogen Phosphorus Potassium loog leaves Duncan's Multiple Range Test at 1% level Column 3 Spots per 0 g. of leaves, February Column 8 Spots per 0 g. of leaves, June Column 4 Yield of leaves in grams, February Column Nitrogen content of leaves in % dry weight, February, Column 7 Potassium content of leaves in % dry weight, February

3 HARKNESS AND REYNOLDS: PHYTOPHTHORA PHILODENDRON 477 quite close to those of the applied solutions, the leaf analyses should be representative of growth to be expected from the applied concentrations. It is important to make soil analyses since some batches of peat moss seem to affect the solu tion concentration greatly. In general, the po tassium content increases and the ammonium nitrogen decreases as the solution remains in the flats. On February the plants were inoculated by spraying upper and lower leaf surfaces with a suspension of zoospores, zoosporangia and mycelial fragments of the fungus. The inoculum was prepared by growing the fungus on oat meal agar. Plugs from these oatmeal cultures were placed on plates of water agar until many zoospores were formed. The medium from the water agar plates was chopped in a blender with a minimum of water and diluted to form inoculum. Phytophthora leaf spots were counted nine days after inoculation, and then the leaves and stems were harvested and weighed. All compari sons of the factors affecting disease incidence were based on the number of leaf spots per 0 grams of leaf and stem. Mature leaves without petioles were selected for leaf analyses. Results and Discussions The results of the 194 tests are shown in Table 2. Column 3 shows spots per 0 grams of tissue on February 19, 194. After harvesting the tops of the plants, they were allowed to grow again and reinoculated in June. Column 8 shows that the results of the second inoculation were similar to those of the first. As shown in the footnotes to Table 2, the only variations in order of treatments between the two inoculations were FIGURE I EFFECT OF SOLUTION NCENT RATON ON DISEASE INQDENCE UJ I Q N, PPM IN SOLUTION K, PPM IN SOLUTION

4 478 FLORIDA STATE HORTICULTURAL SOCIETY, 194 not significant according to Duncan's Multiple Range Test. Column 4 gives the total yields for November 14 plus February 19. The individual harvests are not listed in the table but they were nearly pro portional to the totals. Columns, and 7 of Table 2 give the nitro gen, phosphorus and potassium contents of the leaves in percent dry weight. The calcium con tent averaged 2.0% and neither the phosphorus nor the calcium seemed to be affected by the treatments. Leaf analyses were not made of the June samples. Figure 1 shows the number of spots per 0 g. of leaf plotted against the nitrogen and against the potassium content of the solutions. Figure 2 shows the growth of the plants plotted against the same variables. The greatest number of spots and the greatest growth occurred at intermediate amounts of nitrogen. Three hundred ppm of ni trogen decreased the growth considerably and greatly reduced the incidence of the disease. The optimum growth was at ppm of potassium, the maximum used, but the disease was not greatly affected by the potassium content of the solutions. Figure 3 compares the potassium content of leaves with the nitrogen and potassium contents of the solutions. It is seen that ppm of nitro gen greatly depressed the potassium content of the leaves and in general the potassium content of the leaves was affected as much by the nitro gen content of the solution as by the potassium content. On the other hand, an inspection of the data shows that the nitrogen content of the leaves was not greatly affected by the potassium content of the solutions. Figure 4 compares plant growth with the 20 FIGURE 2 EFFECT OF SOLUTION NCENTRATION ON GROWTH OF PLANTS 2OO N, PPM IN SOLUTION K, PPM IN SOLUTION

5 HARKNESS AND REYNOLDS: PHYTOPHTHORA PHILODENDRON 479 nitrogen and potassium content of the leaves. There was no consistent relation with nitrogen content, but it is seen that the greatest growth was obtained when the potassium content of the leaves was more than 4.0%. Figure compares disease incidence with the nitrogen and potassium contents of the leaves. There was a rather definite decrease in number of spots as the potassium content decreased but unlike the solution concentration data shown in Figure 1, the leaf nitrogen increased only slightly as the disease incidence decreased. A comparison of Figure with Figure 3 and Figure 1 indicates that high nitrogen in the solution reduced disease incidence because it hindered absorption of po tassium by the plants. Some of the results of these experiments are highly significant but they should not be extra polated to other conditions. For example, there was no variation in the phosphorus content al though the preliminary experiments in 193 indicated that very low phosphorus combined with normal nitrogen and potassium (0.3% phosphorus in the leaves as compared with a minimum of 0.248% obtained in February. 194) reduced the incidence of the disease. On the other hand, in those preliminary experiments, a nearly unfertilized treatment with 1.13% N, 0.132% P and 1.38% K in the leaves, had twice as many spots as any of the fertilized treatments. This potassium level was much below the mini mum of 2.3% in February 194 and yet, ac cording to Figure, the disease incidence in creased with the potassium content in the range from 2.3 to.40%. Summary With nutrient solutions containing 1 ppm of phosphorus and ranging from to ppm of nitrogen and to ppm of potassium, the in cidence of Phytophthora leaf spot in Philodendron FIGURE 3 EFFECT OF SOLUTION NCENTR ATION ON POTASSIUM 7.0 NTENT OF LEAVES.0 to.0 UJ N, PPM IN SOLUTION K, PPM IN SOLUTION

6 4 FLORIDA STATE HORTICULTURAL SOCIETY, 194 oxycardium did not appear to be affected by the potassium concentration in the solution. The di sease incidence was much the lowest in treatments receiving ppm of nitrogen. Optimum growth was obtained with somewhat lower concentra tions of nitrogen. Leaf analyses showed that the high nitrogen concentration of soil did not greatly increase the nitrogen content of the leaves but caused a marked reduction of the potassium content. Based on leaf analyses, rather than con centrations of the soil solution, the incidence of the leaf spot increased as the potassium content increased but was not greatly affected by nitrogen content. In nursery practice, it appears that the inci dence of Phytophthora leaf spot might be reduced by using small or moderate amounts of potassium fertilizer combined with more nitrogen than is required for maximum growth of the plant. LITERATURE CITED 1. Doolittle, S. P Tomato diseases. Farmers Bul letin No. 1934, p. 30. United States Dept. of Agric. 2. McFadden, L. A Florida Agricultural Experi ment Station Annual Report, p Gainesville, Fla. 3. Nicholls, C. A., C. N. Smith, and D. L. Brooke A survey of the Florida foliage plant industry. Univ. of Fla. Agric. Exp. Sta. Bulletin No. 1, p FIGURE 4 RELATION BETWEEN LEAF ANALYSIS AND GROWTH PPM N o IN SOLUTION o a X ui o'.x a Q. I x O % N IN LEAVES % K IN LEAVES

7 KERR: MITES ON ORNAMENTALS 481 LJ FIGURE o o RELATION BETWEEN L EAF ANALYSIS AND DISEASE INCIDENCE PPM N IN SOLUTION <? O 40 O 1 X o o t 8 m.» 0 o o - Q a: 0. H- O Q i X X i i i i % N IN LEAVES % K IN LEAVES SOME TETRANYCHID MITES ON FLORIDA ORNAMENTAL CROPS, AND LABORATORY STUDIES OF THEIR S. H. Kerr Department of Entomology SUSCEPTIBILITY Florida Agricultural Experiment Station Gainesville Introduction Spider mite infestations have not, in general, been severe problems in the culture of ornamen- Florida Agricultural Experiment Stations Journal Series No lldentified by H. B. Boudreaux, Louisiana State Univer sity, Baton Rouge, Louisiana. 2ldentified by M. H. Muma, Citrus Experiment Station, Lake Alfred, Florida. TO MITICIDES tal plants in Florida. Most nurserymen report that they are able to achieve control when neces sary. The widespread Florida practice of grow ing ornamentals out-of-doors or in variously shaded beds appears to have prevented the de velopment of extreme degrees of resistence to miticides reported elsewhere in mites infesting greenhouse-grown ornamentals. Chrysanthemums and roses are examples of ornamental crops in Florida on which mites are regularly found. Success in controlling mites with a given miticide on these hosts may be er ratic. Sometimes the cause appears to be that different species of mites are being encountered rather than that a given species has developed a