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Tropical Plant "Avocado"
Persea americana
The
avocado, unflatteringly known in the past as alligator pear,
midshipman's butter, vegetable butter, or sometimes as butter
pear, and called by Spanish-speaking people aguacate,
cura, cupandra, or palta; in Portuguese,
abacate; in French, avocatier; is the only important
edible fruit of the laurel family, Lauraceae. It is botanically
classified in three groups: A), Persea americana
Mill. var. americana (P. gratissima Gaertn.),
West Indian Avocado; B) P. americana Mill. var. drymifolia
Blake (P. drymifolia Schlecht. & Cham.), the Mexican
Avocado; C) P. nubigena var. guatemalensis
L. Wms., the Guatemalan Avocado.
Avocado Description
The
avocado tree may be erect, usually to 30 ft (9 m) but sometimes
to 60 ft (18 m) or more, with a trunk 12 to 24 in (30-60 cm)
in diameter, (greater in very old trees) or it may be short
and spreading with branches beginning close to the ground.
Almost
evergreen, being shed briefly in dry seasons at blooming time,
the leaves are alternate, dark-green and glossy on the upper
surface, whitish on the underside; variable in shape (lanceolate,
elliptic, oval, ovate or obovate), 3 to 16 in (7.5-40 cm)
long. Those of the Mexican race are strongly anise-scented.
Small, pale-green or yellow-green flowers are borne profusely
in racemes near the branch tips. They lack petals but have
2 whorls of 3 perianth lobes, more or less pubescent, and
9 stamens with 2 basal orange nectar glands.
The
fruit, pear-shaped, often more or less necked, oval, or nearly
round, may be 3 to 13 in (7.5-33 cm) long and up to 6 in (15
cm) wide. The skin may be yellow-green, deep-green or very
dark-green, reddish-purple, or so dark a purple as to appear
almost black, and is sometimes speckled with tiny yellow dots,
it may be smooth or pebbled, glossy or dull, thin or leathery
and up to 1/4 in (6 mm) thick, pliable or granular and brittle.
In some fruits, immediately beneath the skin there is a thin
layer of soft, bright-green flesh, but generally the flesh
is entirely pale to rich-yellow, buttery and bland or nutlike
in flavor. The single seed is oblate, round, conical or ovoid,
2 to 2 1\2 in (5-6.4 cm) long, hard and heavy, ivory in color
but enclosed in two brown, thin, papery seedcoats often adhering
to the flesh cavity, while the seed slips out readily. Some
fruits are seedless because of lack of pollination or other
factors.
Avocado Origin
and Distribution
The
avocado may have originated in southern Mexico but was cultivated
from the Rio Grande to central Peru long before the arrival
of Europeans. Thereafter, it was carried not only to the West
Indies (where it was first reported in Jamaica in 1696), but
to nearly all parts of the tropical and subtropical world
with suitable environmental conditions. It was taken to the
Philippines near the end of the 16th Century; to the Dutch
East Indies by 1750 and Mauritius in 1780; was first brought
to Singapore between 1830 and 1840 but has never become common
in Malaya. It reached India in 1892 and is grown especially
around Madras and Bangalore but has never become very popular
because of the preference for sweet fruits.
It
was planted in Hawaii in 1825 and was common throughout the
islands by 1910; it was introduced into Florida from Mexico
by Dr. Henry Perrine in 1833 and into California, also from
Mexico, in 1871. Vegetative propagation began in 1890 and
stimulated the importation of budwood of various types, primarily
to extend the season of fruiting. Some came from Hawaii in
1904 (S. P. I. Nos. 19377-19380).
Now
the avocado is grown commercially not only in the United States
and throughout tropical America and the larger islands of
the Caribbean but in Polynesia, the Philippines, Australia,
New Zealand, Madagascar, Mauritius, Madeira, the Canary Islands,
Algeria, tropical Africa, South Africa, southern Spain and
southern France, Sicily, Crete, Israel and Egypt.
Though
the Spaniards took the avocado to Chile, probably early in
the 17th Century and it was planted from the Peruvian border
southward for over 1000 mi (1,600 km) actual commercial plantings
were not established until California cultivars were introduced
about 1930 into two areas within 100 mi (160 km) of Santiago
where the industry is now centered.
The
first trees were planted in Israel in 1908, but named cultivars
('Fuerte' and 'Dickinson') were not introduced until 1924.
These aroused interest in the feasibility of the crop for
the southern half of the coastal plain and the interior valleys,
and development of the industry has steadily gone forward,
except for a period in the 1960's when much planting stock
was destroyed because of marketing problems. In 1979, Israel
produced 33,000 tons (30,000 MT) and exported 28,600 tons
(26,000 MT).
In
just the last few years, New Zealand has launched a program
to expand commercial production, especially in the Bay of
Plenty area, with protection from wind and frost, with a view
to becoming a major exporter of avocados.
California
produced 265 million lbs (12,045 MT) in 1976; 486 million
lbs (22,090 MT) in 1981. The Florida avocado potential is
estimated at 150 million lbs (6,818 MT). Both states suffer
fluctuations because of the impact of periodic freezes, droughts,
high winds or other seasonal factors.
Presently,
Mexico, with 150,000 acres (62,500 ha) is the leading producer-267,786
tons (243,000 MT); the Dominican Republic is second-144,362
tons (131,000 MT); U.S.A. (California and Florida combined)
with 52,000 acres (21,666 ha), third-131,138 tons (119,000
MT); Brazil is fourth-128,934 tons (117,000 MT). Israel, with
16,000 acres (6,666 ha), is fifth; and South Africa sixth.
Half of California's plantings are in San Diego County close
to Mexico.
As
an exporter, Mexico again leads, followed by California, Israel,
South Africa and Florida, in that order. Nearly all of Brazil's
crop is consumed domestically.
Avocado Varieties
WEST
INDIAN race: Florida avocados were at first mainly of the
summer fruiting West Indian race, but these had to compete
commercially with similar fruits imported from Cuba, and growers
sought other cultivars maturing at a later season. This led
to the development of West Indian X Guatemalan hybrids. The
cessation of trade with Cuba in the early 1960's brought about
a shift back to summer cultivars in new groves to fill the
gap. The majority of the avocados grown in the West Indies,
Bahamas and Bermuda and the tropics of the Old World are still
of the West Indian race. The skin is leathery, pliable, non-granular,
and the flesh low in oil. The leaves are not aromatic. The
following are the most prominent of early and more recent
West Indian cultivars which have played an important role
in the development of the avocado industry in Florida and
elsewhere. New selections appear from time to time that may
have special adaptability to certain locales or conditions.
'Butler'
(a USDA selection in Florida; fruited in 1909, propagated
from 1914 to 1918) pear shaped; medium-large; skin smooth;
seed of medium size, tight in the cavity. Season: Aug.-Sept.
No longer grown in Florida. Cultivated in Puerto Rico.
'Fuchs'
('Fuchsia') (seed of unknown origin planted in Homestead,
Florida, in 1910; propagated commercially in 1926); pear shaped
to oblong, sometimes with a neck; of medium size; skin smooth;
flesh pale greenish-yellow; 4 to 6% oil; seed loose. Season:
earlyJune-Aug.; a poor shipper. Tree not very productive in
Florida; no longer popular in commercial groves.
'Maoz'
(a seedling selected from a plot near Maoz, Israel); pear-shaped;
of medium size; skin rough, leathery, violet-purple when ripe;
flesh sweetish and very low in oil. Season: medium-late (Oct.).
Tree is an alternate bearer but is fairly small, highly salt-tolerant;
used in Israel as rootstock on either saline or calcareous
soils.
'Pollock'
(originated in Miami before 1896; commercially propagated
in 1901); oblong to pear shaped; very large, up to 5 lbs (2.27
kg); skin smooth; flesh green near skin, contains 3 to 5%
oil; seed large, frequently loose in cavity. Season: early
July to Aug. or Oct. Shy-bearing and too large but of superior
quality.
'Ruchle'
(a seedling of Waldin planted at the Agricultural Research
and Education Center, Homestead, in 1923; first propagated
in 1946); pear-shaped; of medium size, 10 to 20 oz (280-560
g); flesh low in oil (2-5%). Season begins in July in Florida;
Jan. in Queensland. Heavy bearer in Florida.
'Susan'
(evaluated by California Avocado Society January 2, 1975;
patented but patent has now expired); pear-shaped; of medium
size, averaging 8 to 10 oz (227-283 g); skin light-green smooth,
thin, peels well; flesh pale cream-color, of bland flavor;
ripens unevenly with darkening spots; has slight tendency
to turn dark when cut; not attractive; of only fair quality;
seed large, loose; coat adheres to seed. Season: early fall;
short. Tree of medium size; grown commercially only in the
San Joaquin Valley because of its cold hardiness.
Many
local and introduced cultivars representing all 3 races are
being grown and evaluated at the experimental station at Minas
Gerais, Brazil. A large collection is also maintained in Bahia.
The U.S. Department of Agriculture has an international repository
of 170 clones in Miami.
In
general, small to medium-sized fruits are best for commercial
production and especially for metropolitan markets. Large
fruits are suitable for local use especially by large families.
Smooth, thin or fairly thin, pliable, green skin is preferred
by the consumer. The flesh should be virtually fiberless and
of agreeable flavor and, for the dieter, of low oil content.
The seed must be small and tight so as not to bruise the flesh
during handling and shipping. The seed coats ought to adhere
to the seed and not to the cavity. The fruit should ship well
and stand cold storage. The tree should be of moderate height,
slender enough to permit judiciously close planting without
crowding. It should bear at an early age and regularly but
not so heavily as to suffer die back, and, of course, should
be disease-, insect-, and, in subtropical areas, cold-resistant.
Cold-resistant cultivars stand cold-storage better than cold-sensitive
cultivars.
Avocado Pollination
Many
isolated avocado trees fail to fruit from lack of pollination.
Commercial growers are careful to match Class A cultivars
whose flowers will receive pollen in the morning with Class
B cultivars that release pollen in the morning and every grower
must be sure to include compatible pollinators in his grove.
Bulletin 29 (1971) of the Ministry of Agriculture in Guatemala
tabulates the flowering periods (varying from August to April)
of 48 introduced and locally selected cultivars, and the hours
of the day when each is receptive to or shedding pollen.
Avocado Climate
The
West Indian race requires a tropical or near tropical (southern
Florida) climate and high atmospheric humidity especially
during flowering and fruitsetting. The Guatemalan race is
somewhat hardier, having arisen in subtropical highlands of
tropical America, and it is successful in coastal California.
The Mexican race is the hardiest and the source of most of
California avocados. It is not suited to southern Florida,
Puerto Rico or other areas of similar climate. Temperatures
as low as 25ºF (-4ºC) do it little harm. In areas of strong
winds, wind-breaks are necessary. Wind reduces humidity, debydrates
the flowers and interferes with pollination, and also causes
many fruits to fall prematurely.
Avocado Soil
The
avocado tree is remarkably versatile as to soil adaptability,
doing well on such diverse types as red clay, sand, volcanic
loam, lateritic soils, or limestone. In Puerto Rico, it has
been found healthier on nearly neutral or slightly alkaline
soils than on moderately or highly acid soils. The desirable
pH level is generally considered to be between 6 and 7, but,
in southern Florida, avocados are grown on limestone soils
ranging from 7.2 to 8.3. Mexican and Guatemalan cultivars
have shown chlorosis on calcareous soils in Israel. The tree's
primary requirement is good drainage. It cannot stand excessive
soil moisture or even temporary water-logging. Sites with
underlying hardpan must be avoided. The water table should
be at least 3 ft (.9 m) below the surface. Salinity is prejudicial
but certain cultivars (see 'Fuchs-20' and 'Maoz') have shown
considerable salt-tolerance in Israel. Avocados grafted onto
'Fuch-20' rootstocks and irrigated with water containing 380
to 400 ppm C1 performed well in a commercial orchard. In the
Rio Grande Valley of Texas, cultivars of the Mexican race
must be grafted onto salt tolerant West Indian rootstocks.
Avocado Propagation
Normally,
avocado seeds lose viability within a month. 'Lula' seeds
can be stored up to 5 months if placed in non-perforated polyethylene
bags and kept at 40ºF (4.4ºC), thus indicating that it may
be possible to successfully store seeds of other cultivars
ripening at different seasons for later simultaneous planting.
Fresh seeds germinate in 4 to 6 weeks, and many people in
metropolitan areas grow avocado trees as novelty house plants
by piercing the seed partway through with toothpicks on both
sides to hold it on the top of a tumbler with water just covering
1/2 in (1.25 cm) of the base. When roots and leaves are well
formed (in 2 to 6 weeks), the plant is set in potting soil.
Of course, it must be given adequate light and ventilation.
In nurseries, seeds that have been in contact with the soil
are disinfected with hot water. Experiments with gibberellic
acid and cutting of both ends of the seed with a view to achieving
more uniform germination have not produced encouraging results.
Seedlings will begin to bear in 4 or 5 years and the avocado
tree will continue to bear for 50 years or more. Some bearing
trees have been judged to be more than 100 years old.
In
Australia, seeds planted in early fall germinate in 4 to 6
weeks; if planted later, they may remain dormant all winter
and germinate in early spring. Seedlings should be kept in
partial shade and not overwatered. While many important selections
have originated from seeds, vegetative propagation is essential
to early fruiting and the perpetuation of desirable cultivars.
However, seedlings are grown for rootstocks.
For
many years, shield budding was commonly practiced in Florida,
but this method requires considerable skill and experience
and is not successful with all cultivars. Therefore, it was
largely replaced by whip, side-, or cleft-grafting, all of
which make a stronger union than budding.
In
the past, seedlings were grafted when 18 to 36 in (45-90 cm)
high. It is now considered far better to graft when 6 to 9
in (15-23 cm) high, making the graft 1 to 3 in (2.5-7.5 cm)
above ground level. West Indian rootstocks are desirable for
overcoming chlorosis in avocados in Israel.
Avocado
cuttings are generally difficult to root. Cuttings of West
Indian cultivars will generally root only if they are taken
from the tops or side shoots of young seed rings. But etiolated
cuttings (new shoots) from gibberellin treated hardwood and
semi hardwood cuttings of 'Pollock' as well as 'Lula' have
been rooted with 50-60% success and, when treated with IBA,
66-83% success under mist in Trinidad. Cuttings of 'Fuchs-20'
have rooted under mist with 40 to 50 or even 70% in Israel.
Cuttings of 'Maoz' have rooted at the rate of 60% by a special
technique developed in California. An Israeli selection, 'G.A.
13' has given 70 to 90% success in rooting cuttings under
mist for the purpose of utilizing them as rootstocks in saline
and high lime situations. Air-layering is sometimes done to
obtain uniform material uninfluenced by rootstock, for research
on specific problems. Degree of success depends on the cultivar
(those of the Mexican race rooting most quickly), and air-layering
is best done in spring and early summer.
At
times, mature avocado groves are top worked to change from
an unsatisfactory cultivar, or one declining in popularity,
to a more profitable one, or an assortment of cultivars for
different markets. In 1957, 2,700 ";obsolete"; avocado trees
in Ventura, California, were being grafted (top-worked) to
mainly 'Hass', some to 'Bacon' and 'Rincon'. This procedure
may involve thousands of trees in a given region. It is done
in December and January in Florida.
Inasmuch
as avocado roots are sensitive to transplanting, it is now
considered advisable to raise planting material in plastic
bags which can be slit and set in the field without disturbing
the root system.
Avocado Spacing
Spacing
is determined by the habit of the cultivar and the character
of the soil. In light soil, 25 x 25 ft (7.5x7.5 m) may be
sufficient. In deep, rich soil, the tree makes its maximum
growth and a spacing of 30 or 35 ft (9.1 or 10.7 m) may be
necessary. If trees are planted so close that they will ultimately
touch each other, the branches will die back. Some growers
plant 10 to 15 ft (3-4.5 m) apart initially and remove every
other tree at 7 to 8 years of age. If the surplus trees are
not bulldozed but just cut down leaving a stump, application
of herbicide may be needed to prevent regrowth. Ammonium sulfamate
has been proven effective. In modernized plantings, space
between rows is necessary for mechanical operations.
Holes
at least 2 ft (0. 6 m) deep and wide are prepared well in
advance with enriched soil formed into a mound. After the
young plant is put in place a mulch is beneficial, weeds should
be controlled, and watering is necessary until the roots are
well established. Generally small amounts of fertilizer are
given every 2 months with the amount gradually increasing
until fruiting begins. Bearing trees need, on the average,
3 to 4 lbs (1 1/2-2 kg) 3 times a year, beginning when the
tree is making vegetative growth. No fertilizer should be
given at blooming time; one must wait until the fruits are
firmly set. Nitrogen has the greatest influence on tree growth,
its resistance to cold temperatures, and on fruit size and
yield. Fertilizer mixes vary greatly with the type of soil.
Mineral deficiencies determined by leaf analysis, are usually
remedied by foliar spraying. Magnesium deficiency was formerly
a serious handicap to avocado growers in Florida and Kenya.
In California, zinc deficiency has been corrected by applying
zinc chelates or zinc sulfate to the soil instead of spraying
the foliage.
Keeping
the upper soil moist has been greatly facilitated by drip
irrigation, which also may carry 80% of the fertilizer requirement.
Because
some cultivars tend to grow too tall for practical purposes,
commercial growers cut trees back to 16 or 18 ft (4.8-5.4
m), let them grow back to 30 ft (9.1 m) and top them again.
But decapitation is not a perfect remedy because the tendency
of the avocado tree is to grow a new top very quickly. Recently
it has been found that the growth-inhibiting chemical, TIBA
(triiodobenzoic acid) slows down terminal growth and encourages
lateral shoots. A system of pruning to encourage lower branching
is being tried on 'Lula' in Martinique.
Avocado
branches frequently need propping to avoid breaking with the
weight of the developing fruits.
Some
growers find it profitable to interplant bananas until the
avocado trees reach bearing age.
Avocado Maturity
and Harvesting
Avocados
will not ripen while they are still attached to the tree,
apparently because of an inhibitor in the fruit stem. Homeowners
usually consider the entire crop pickable when a few mature
(full grown) fruits have fallen. This is not a dependable
guide because the prolonged flowering of the avocado results
in fruits in varying stages of development on the tree at
the same time. The largest fruits, of course, should be picked
first but the problem is to determine when the largest are
full grown (perfectly mature for later perfect ripening).
If picked when full grown and firm, avocados will ripen in
1 to 2 weeks at room temperature. If allowed to remain too
long on the tree, the fruits may be blown down by wind and
they will be bruised or broken by the fall.
Florida
maturity standards for marketing have been determined by weight
and time of year for each commercial cultivar so that immature
fruits will not reach the market. Immature fruits do not ripen
but become rubbery, shriveled and discolored. Most West Indian
cultivars will ripen properly if picked when the specific
gravity becomes 0. 96 or lower, but 'Waldin' is fully mature
when the specific gravity is still above 0.98. Guatemalan
and Guatemalan X West Indian cultivars generally are harvest-mature
when the specific gravity is 0.98 or lower. In California,
physiological maturity of 'Bacon', 'Fuerte,' 'Hess' and 'Zutano'
has been determined by measurement of length, diameter and
volume, but dry weight, correlating with oil content, is considered
a better maturity index. California law has, since 1925, required
a minimum of 8 % oil, but oil content varies greatly among
cultivars and also the climatic region where the fruit is
grown. Some people complain that the 8% standard is too low
for some cultivars. Maximum flavor of 'Fuerte' develops when
the fruit is harvested at an oil content of 16%. Therefore,
a minimum dry weight standard of 21 % has been recommended.
Formerly,
avocados were detached by means of a forked stick and allowed
to fall, but this causes much damage and loss. Nowadays harvesters
usually use clippers for lowhanging fruits and for those higher
up a long handled picking pole with a sharp ";V"; on the metal
rim to cut the stem and a strong cloth bag to catch the fruit.
Gloves are worn to avoid fingernail scratches on the fruit.
In California, studies have been made of the effects of hand
clipping (leaving stem on), hand snapping (which removes the
stem), tree-shaking, and limb shaking (which removes the stem
from some of the fruits). All methods are acceptable if the
stem scar is waxed on stemless fruits to avoid weight loss
before ripening at which time the stem detaches naturally.
In Australia, some growers are using hydraulic lifts to facilitate
hand-picking. A tractor fitted with a triple-decked picking
platform has been adopted by some large growers in Chile.
Efforts to develop dwarf avocado trees by means of sandwich
interstocks from low growing types have been going on in California
since 1964.
Avocados
must be handled with care and are packed and padded in single
or double-layer boxes or cartons for shipment. A special ";Bruce
box";, holding 32 lbs (14.5 kg) is used for large fruit. The
fruits may be held in position in molded trays.
Avocado Pests
and Diseases
Avocados
have no major insect enemies in Florida but migrating cedar
waxwings feed on leaves, flowers and very young fruits and
the fruits are commonly attacked by squirrels, rats and mice.
The avocado red mite, Oligonychus yothersi; is the
most common predator on the leaves in some groves and not
in others. Red-banded thrips, Selenothrips rubrocinctus,
the greenhouse thrips, Heliothrips haemorrhoidalis,
and red-spider, Tetranychus mytilaspidis, may feed
on avocado leaves and blemish the fruits from time to time.
There are several scales also which may feed on foliage, especially
the Florida wax scale, Ceroplastes floridensis, the
pyriform, or soft white, scale, Protopulvinaria pyriformis,
Dictyospermum scale, Chrysomphalus dictyospermi; and
the black scale, Saissetia oleae. Among two dozen other
minor pests in Florida are the citrus mealybug, Pseudococcus
citri and avocado mealybug, P. nipae. Stinkbugs
may prick the fruits leaving little dents in the skin coupled
with gritty areas at the same locations inside.
In
California, 2 lepidopterous pests, Amorbia cuneana
and the omnivorous looper, Sabulodes aegrotata,
when present in large numbers, cause severe defoliation and
fruit-scarring. Biological control is being achieved by release
of the egg parasite, Trichogramma platneri;
which is now commercially available to growers. Since 1949,
the orange tortrix (a leaf roller), Argyrotaenia citrana,
has been increasing as a menace to the avocado in California,
the larvae feeding on twigs, terminal buds and foliage, flowers,
and fruits. Since the pest requires shaded areas, it is best
controlled by thinning out a close-planted grove or top-working
to less susceptible cultivars.
The
fruit-spotting bug, Amblypelta nitida, and banana
spotting bug, A. Iutescens, are important pests requiring
control in Queensland. The Mediterranean fruit fly is a major
hazard in Israel, but very thick-skinned fruits such as 'Anaheim'
are not attacked. The Queensland fruit fly, Dacus tryoni;
seriously damages only Mexican cultivars or Guatemalan X Mexican
hybrids in Australia. In 1971, a nematode survey in Bahia,
Brazil, revealed 9 genera of known or suspected parasitic
nematodes associated with avocado tree decline. Israeli avocado
growers are seeking and testing means of biological control
of the more serious of the 3 dozen insects and mites preying
on the crop in that country. In Mexico, the avocado weevil,
Heilipus lauri; tunnels into the seeds.
The
major disease of avocados in South and Central America and
some islands of the West Indies, in California, Hawaii, and
various other areas, is root-rot caused by the fungus, Phytophthora
cinnamomi, which is being combatted by the use of strict
sanitary procedures and resistant rootstocks, especially 'Duke'.
At the University of California, Riverside, over 750 seedlings
and cuttings were being tested for root-rot resistance in
1976 and 1977 and the most promising tried out for grafting
compatibility with commercial cultivars. Also, soil fumigation
experiments with methyl bromide and newly developed chemicals
were being carried forward. The disease has been so devastating
in the high rainfall areas of New South Wales and Queensland
that plantings have expanded into the semi-arid Murray Valley
in the hope of avoiding it. In New Zealand, it is not a problem
on deep, volcanic soils, but occurs on shallow, heavier soils.
It was allegedly introduced into Chile with balled trees from
California and vigorous measures are being taken to control
it.
Mushroom
root-rot from Clitocybe tabescens may occasionally
occur. Cercospora spot (brown spots on the leaves and fruits),
caused by the fungus, Cercospora purpurea, may
cause cracks in affected areas of the skin and thus allow
entrance of the anthracnose fungus, Colletotrichum
gloeosporioides, which invades and spoils the flesh.
Glomerella cingulata is an important source
of anthracnose in Queensland. Some cultivars are subject to
scab which is readily controlled by copper sprays.
More
than 30 other pathogens are variously responsible for wood
rot, collar rot, dieback, leafspot, stem-and rot of fruit,
branch canker, and powdery mildew. Sunblotch viroid cripples
young trees and damages fruits in California and Israel. So
far, it is unknown in New Zealand. Stems of young trees may
be affected by sunburm, and hot, dry winds cause tipburm of
leaves. The avocado tree may show copper or zinc deficiency
or tipburm from an excess of mineral salts.
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