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Introduction
Calla Lily Bulbs are indeed a high value crop, and with this high value comes increased risk of disease during the calla
lily production cycle. Stress management is the key to the successful production of colored calla lilies. By minimizing stress
to the calla lily plant, the plant's susceptibility to disease is also minimized and a profitable crop prevails.
Calla lily soft rot, caused by the Erwinia caratovera bacterium, is the main limiting factor in calla production. Once a calla
lily tuber is infected, there is no chemical or other control method that will rid the tuber of this bacterium. Infection from
this disease is a secondary infection, however, primary infection usually occurs via pythium, rhizoctoria, phytopthera, or fusarium.
These fungal pathogens stress or damage the calla lily bulb and allow the invasion of soft rot. Therefore, all management practices
should aim at preventing primary infections from fungal pathogens and to culture a strong, healthy calla lily plant.
Detailed management of air and soil temperature, media selection, water availability, plant nutrition, and a preventative disease
program are essential to the culture of calla lily bulbs.
Temperature
Temperature is vital to the performance of calla lily tubers. The ideal day/night temperature regime is 65°-75°F/55°-65°F.
Recommended soil and media temperature is 65°F. It is important to keep soil temperature below 75°F, because higher temperatures
cause stress and contribute to disease. Shade cloth, greenhouse paint, or soil mulches can all help to control heat stress and
to improve flower quality.
Soil Media
Calla lilies grow best in loose, free draining soils with a pH of 6-6.5. This applies to both natural soil and soilless
growing media. Crop rotations or periods of fallow are important to prevent the buildup of Erwinia. If drainage is a problem,
plant in raised beds.
Irrigation
Calla lilies require an ample supply of water. Until leaves unfurl, water uptake is not real large, but the tuber requires
moisture to grow and should be kept damp. At the vegetative leaf growth phase, the plant grows quickly and water uptake increases
greatly. Even, consistent watering is important as callas are easily stressed by too little or too much water. Drip and overhead
irrigation can be used. At the end of the growing cycle when plant senescence begins, water should be withheld.
Nutrition
In a soilless growing media, a balanced three to four month slow release fertilizer plus minors can be incorporated or
top-dressed. The addition of calcium in the media may help with disease control and is a good precaution. Weekly fertigation also
works well, beginning at leaf emergence and extending until first bloom with approximately 200 ppm N using 20-10-20.
The addition of calcium in natural soils is thought to help subdue the severity of soft rot as well. Upon the results of a soil
test, the addition of NPKMg plus trace elements is used to supply the necessary nutrients for plant growth. Organic matter helps
the nutrient and water holding capacity of soils, and is greatly beneficial to tuber growth. Excessive use of N will result in
weak stems and leaf growth, as well as increased susceptibility to soft rot.
Planting
In field production, one-meter wide beds are generally used. Planting densities for 3cm, 4cm, and 6+cm tubers are 35, 25,
and 15 tubers/meters2 respectively. Ensure that tubers are planted upright and covered with 2 in. of soil. Larger tubers (2 in.+)
should be planted 3-4 in. deep to keep tubers out of hot soil during the heat of the summer.
Pre-Plant Sprays/Dips
All flowering stock should be dipped or sprayed with Giberellic acid (GA3) or Promalin (GA4+7) prior to planting. Promalin
is the preferred compound as less deformed flowers will be produced. The use of giberellic acid can increase flower initiation
by up to 300%. For spraying, use 100 ppm Promalin or 125 ppm GA3. For dipping reduce Promalin to 75 ppm or GA3 to 100 ppm. It
is also beneficial prior to planting to dip or spray with a suitable fungicide and bactericide solution (e.g. Kocide, Champ II
flowable).
Weed Control
Calla plants are tolerant to a range of herbicides in the aid of weed control. A pre-plant eradication of perennial weeds
with Roundup and a pre-emergent application of Simazine or Surflan is suggested. Weed control can be extended with an additional
post-emergent application of Simazine or Surflan. Weed control in the latter half of a calla's production season is not a problem
as the dense foliage helps to reduce weed growth.
Insects
Thrips and aphids are the main pests of callas. Control is important to prevent the spread of viruses and fungi. A preventative
spray program is recommended as flower spikes emerge, to be repeated at 7-10 day intervals during flower production. Insect control
via chemical application may be necessary following flowering as well due to increased disease pressure and its concurrent spread
with the presence of thrips and aphids.
Fungal Pathogens
As previously mentioned, calla lily tubers can be attacked by a range of fungal pathogens including pythium, rhizoctoria,
phytopthera, or fusarium, acting as a precursor towards soft rot syndrome. These all attack the root zone and common symptoms
include withering, yellow, rolled up leaves, and a subsequent collapse of the base of the stem. A broad-spectrum drench every
2-4 weeks from planting onwards is crucial to controlling these fungi. In pot production, the use of 1) Agrimycin-17 @ 100-200
ppm; 2) Aliette @ 13 oz/100 U.S. gal.; 3) Chipco 26019 @ 6.5 oz/100 U.S. gal. is the preferred combination of fungicides. In natural
soils, Benlate, Thiram, and Aliette is a good combination for broad-spectrum control.
Fungal spotting on flowerheads during humid and/or rainy weather is a common problem. Protectant fungicidal sprays should be used
every 7-10 days during problem periods. Be careful of residues on flowers.
Flower Harvest
Harvest flowers in the cool of the morning or evening. To ensure the longest possible stem length, flowers are pulled rather
than cut. Coolstore flowers at 45-48°F for 8-12 hours in a solution of flower preserve. This will minimize stem splitting
and rotting, and control post harvest fungal diseases. Re-cut the stems before they are placed in the condition solution. Pre-treated
flowers can be transported dry for up to 3 days.
Tuber Lifting
After plant senescence begins (visible with the onset of yellow and decaying leaves), lifting of the tubers can occur via
hand digging or modified diggers. Great care should be taken during lifting, as young tubers are easily bruised or injured, allowing
diseases to enter. It is also important not to harvest the tubers prior to the beginning of plant senescence. During plant senescence,
tuberization (the replication of new tubers) is completed and the surface of the tuber becomes tougher.
Curing
Wash tubers gently to remove soil. Then dip tubers in an appropriate fungicide solution (e.g. Kocide) for 5-10 minutes.
This helps to eliminate fungal pathogens. Dry tubers within 4-6 hours with air assisted fans if need be. Subsequently cure bulbs
for 3-7 days at 70-80°F, ensuring good air movement. This curing stage is important in forming an outer skin on the tuber,
which acts as a barrier to dehydration and disease. Store tubers in a single layer on mesh trays at 45-50°F for a minimum
of two months prior subsequent plantings.
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