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)A rc Agricultural Research & Education Center
IFAS, University of Florida
5007 60th Street East
Bradenton, FL 33508
Bradenton AREC Research Report BRA1983-15 May 1983
EFFECTS OF MAGNESIUM ON BACTERIAL LEAF SPOT OF PEPPER
S. S. Woltz, J. B. Jones and J. P. Jones1
Observations were made in.1977 (9) that pepper and tomatoes growing under
a restricted magnesium supply versus higher level magnesium nutrition were
relatively free of the effects of a natural infection with Xanthomonas bacter-
ial leaf spot. Hone of the plants in the lower level magnesium treatments
exhibited magnesium deficiency symptoms. A line of research has been pursued
since that time to evaluate the response of pepper and Xanthomonas campestris
pv. vesicatoria to variations in magnesium nutrition and nutritional response
in the disease process.
Magnesium Requirements of Pepper
While the amount of magnesium required by pepper is small relative to
nitrogen, potassium and calcium, the element is nonetheless indispensable.
In order to achieve optimal productivity and quality, magnesium must not be
limiting. Recently matured leaves should contain at least 0.2 to 0.3% magnesium
on a dry weight basis and preferably 50 to 100% more for a safety margin.
Symptoms of magnesium deficiency are clear cut on pepper, with lower or middle
leaves showing a yellowing between the veins beginning toward the margins of
the leaves and progressing toward the interior of the leaf. Veins are not
prominently green, however, as they are for example in iron deficiency.
Nutritional balance must also be taken into account in addition to absolute
amounts of magnesium furnished to avoid antagonizing uptake and nutrition processes
by large excesses of ammonium-nitrogen, calcium or potassium. Under controlled
conditions, magnesium deficiency may be produced in pepper seedlings (8). Mag-
nesium is supplied by dolomitic limestone, magnesium in fertilizers, Florida
soils, and also by irrigation water so that magnesium deficiency is not often
found under field conditions. The roles of magnesium in plants are fairly well
documented, the element being a part of the chlorophyll molecule, essential to
many enzymatic reactions, particularly carbohydrate metabolism, and being
essential to ribosomes plant organelles involved in synthesizing proteins
from amino acids.
Magnesium uptake by roots is a slow process at times because of the low
mobility of the element in the soil; also, once roots are deficient in magnesium,
uptake of the element is limited by the deficiency, necessitating foliar magnesium
spray application. Magnesium deficiency of roots also reduces the size of the
root system and causes a breakdown of pepper roots which become softened and
discolored. Thus, it can be seen that care should be exercised to avoid a
deficiency in the soil.
1Professor of Plant Physiology, Assistant Prdfessor of Plant Pathology and
Professor of Plant Pathology, respectively.
Magnesium Requirements of the Bacterium
The causal agent of bacterial leaf spot of pepper, X. campestris pv.
vesicatoria, requires magnesium (6, 7) for a number of roles including protein
synthesis and many enzymatic reactions. A deficiency of soluble magnesium in
the bacterial environment severely limits cell division and disease production.
Rainfall may accentuate a deficiency for the bacteria by reducing soluble mag-
nesium levels in the soil-plant environment of the bacterium, especially if
magnesium is not available at high levels. The level of magnesium required
by the bacterium for optimum growth in a nutritionally balanced shaken liquid
culture is only about 5 ppm. Responsiveness to much higher levels of magnesium
were found, however, in nutritionally imbalanced media, indicating a capacity of
the element to overcome some nutritional imbalances for the bacterium. Prelimi-
nary evidence developed at the AREC-Bradenton indicates that part of the roles
of copper and streptomycin in control of the pepper leaf spot bacteria may be
an interference with normal magnesium metabolism. At any rate, the bacteria
grown in shaken liquid cultures are able to grow'in the presence of much larger
amounts of copper or streptomycin when magnesium is-also present at high levels.
With low magnesium levels and higher copper or streptomycin, the -bacteria may
fail, depending on the balance, to initiate growth. Unfortunately, most isolates
of X. campestris pv. vesicatoria in Florida have been more resistant to copper
than strains from other regions of the United States. Both copper-sensitive
and copper-resistant strains of the organism respond favorably to magnesium
in terms of increased magnesium-enhanced tolerance to copper. The range of
copper tolerance is much lower for copper-sensitive than copper-resistant
bacteria. That is to say that magnesium only increases copper tolerance of
sensitive strains in liquid cultures to a low level (1-3 ppm) but increases
the tolerance of resistant strains to much higher levels (10+ ppm).
Roles of Magnesium in Bacterial Leaf Spot
The roles of magnesium in the disease process have been studied at the
AREC-Bradenton. Bacteria may be splashed from the soil surface to leaves in
a driving rain and with warm temperatures and high humidity, disease has an
excellent chance of developing. Beginning with the soil and leaf surface,
bacteria face a survival-multiplication situation. Once inside the leaf via
wound or stomates (pores) the bacteria may multiply and exhibit disease-producing
virulence or may fail to produce disease. It is likely that a low level of
magnesium or other essential metabolite will have a negative effect at some
or all of the points listed in the bacteria-distribution and infection process.
Deficiency for the bacterium should be aggravated by lower-level environmental
magnesium and leaching of plant and soil by rain, washing out soluble magnesium
needed by the bacteria. Leaf-leaching studies showed that significant amounts
of magnesium could be removed by washing pepper leaves in water.
In experiments at the AREC-Bradenton, magnesium has been supplied via
dolomitic limestone or magnesium sulfate applied to the soil and by foliar
spray applications of magnesium sulfate or chloride.- Experiments on bacterial
leaf spot with varied magnesium nutrition of pepper have involved natural infec-
tion, injection of inoculum into the leaf and by spraying plants with suspensions
of bacterial cells prepared from laboratory cultures or diseased leaves. Disease
consistently developed at a faster pace with greater incidence and severity when
supplemental magnesium was furnished.
In an early experiment in this series, peppers were grown in quadruplicate
in large plastic pots,.half of which received applications of magnesium sulfate.
The growing medium included Myakka soil, peat and perlite. Noninoculated plants
receiving magnesium had no bacterial leaf spot while those in magnesium-applied
cultures had 4.33J8 leaf area affected with leaf spot. Spray inoculation of low
magnesium plants resulted in 1.81% leaf area affected while high magnesium plants
had 8.44% leaf area affected. The differences were significant at the 5% level
of probability. Further details may be found in reference 9.
In a later experiment (3), peppers were grown in Myakka soil in large wooden
boxes. Cultural treatments included high-calcium and dolomitic limestone with
and'without magnesium chloride spray (Tables 1 and 2). The highest magnesium
levels in leaves were associated with the highest disease levels.
Table 1. Leaf spot development from leaf inoculum injection procedure
Liming MgCl1, Mg in leaves % diseased
material spray / dry wt. leaf area
Dolomite + 1.05 a1 54.9 a
Dolomite -0.74 ab 43.6 ab
High calcium + 0.64 ab 39.0 ab
High calcium -0.52 b 19.7 b
1Means followed by different letters differ at the 5% level of significance.
Table 2. Leaf spot development from foliar spray inoculation
Liming M'gCl2 Mg in leaves % diseased
material spray % dry wt. leaf area
Dolomite + 0.86 al 83.4 a
Dolomite 0.74 ab 54.8 b
High calcium + 0.56 c 59.6 b
High calcium 0.44 c 37.9 b
1Means followed by different letters differ at the 5% level of significance.
Calcium is important in limiting bacterial disease processes (1, 2, 3, 5)
by preserving leaf cell-membrane integrity, retarding leakage of nutrients and
thus retarding the disease development process. Mohanty et al (5) reported
that increasing calcium levels inhibited bacterial leaf blight of rice, caused
by the same bacterial species as the pepper bacterial leaf spot pathogen. They
found that as magnesium nutritional level was increased, disease worsened for 3
rice cultivars inoculated with X. campestris pv. oryzae.
Higher magnesium levels in the leaves supported higher incidence and severity
of bacterial leaf spot of pepper than did lower levels of the element. Bacterial
populations were shown to increase in liquid culture more rapidly with adequate
than inadequate magnesium. As levels of magnesium were increased in liquid cul-
ture of the bacterium, X. campestris pv. vesicatoria was benefited by increasing
tolerance to increasing levels of soluble copper and streptomycin. Pending
acquisition of additional data, especially from field experiment, it is suggested
that growers avoid "luxury" magnesium supplies via soil applications and foliar
sprays. :Adequate magnesium for crop requirements should be furnished. When this
element is. deficient in the plant, further uptake of magnesium and other nutrients
is limited by the deficiency in the roots, therefore, foliar spray applications
become necessary to overcome the deficiency. Care should be taken by soil nutrient
adjustment to avoid a deficiency, but if the deficiency is suspected, foliar appli-
cation of magnesium should be initiated and continued for several weeks to correct
1. Cook, A. A. and R. E. Stall. 1971. Calcium suppression of electrolyte loss
from pepper leaves inoculated with Xanthomonas vesicatoria. Phytopath. 61:
2. Cook, A. A. and R. E. Stall. 1963. Effect of watersoaking on response to
Xanthomonas vesicatoria in pepper leaves. Phytopath. 67:1101-1103.
3. Jones, J. B., S. S. Woltz and J. P. Jones. 1983. The effect of foliar and
soil magnesium application on bacterial leaf spot of peppers. Plant Dis.
67: (in press).
4. Marco, G. M. and R. E. Stall. 1983. Control of bacterial spot of pepper
initiated by strains of Xanthomonas campestris pv. vesicatoria which differ
in sensitivity to copper. Plant Dis. 67: (in press).
5. Mohanty, S. K., P. Ranga Reddy and R. Sridhar. 1982. Effect of calcium and
magnesium on the susceptibility of rice plants to bacterial leaf blight.
Current Sci. 51(6):298-299.
6. Starr, M. P. 1946. The nutrition of phytopathogenic bacteria. I. Minimal
nutritive requirements of the genus Xanthomonas. J. Bacteriol. 51:131-143.
7. Weinberg, E. D. (ed.). 1977. Microorganisms and minerals. Marcel Dekker,
Inc., New York, NY. 492 pp.
8. Woltz, S. S. 1982. Fertilizer requirements of pepper seedlings for trans-
plant production: Symptoms of inorganic nutrient deficiencies. Bradenton
AREC Res. Rept. BRA1982-23.
9. Woltz, S. S. and J. P. Jones. 1979. Effects of magnesium on bacterial spot
of pepper and tomato and or in vitro inhibition of Xanthomonas vesicatoria
by streptomycin. Plant Dis. -3:182-184.