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Fusarium wilts, or yellows, are caused by a number of highly specialized
forms and races of the common soilborne fungus Fusarium oxysporum.
Fusarium wilt diseases attack many popular garden and greenhouse
flowers (Table 1) and are most serious and widespread in Illinois
on aster (China-aster), carnation, chrysanthemum, gladiolus, lily,
and narcissus.
Other specialized forms of Fusarium oxysporum attack
numerous vegetables; a few fruits, field crops, and trees plus a
wide range of other plants (Table 2). Many soils in Illinois and
other crop-growing areas throughout the temperate and tropical regions
of the world contain one or more forms of Fusarium oxysporum.
Once the fungus is introduced into a garden, nursery, greenhouse,
or field, it can live indefinitely in a variety of soil types, independent
of any host plant. This ability to survive eliminates, as an effective
control measure, any normal rotation program or general sanitation.
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Figure
1. Fusarium wilt on cyclamen. Symptoms frequently don't appear
until flowers are mature. (Photo courtesy of A.H. McCain). |
Symptoms of Fusarium wilt are easily confused with root or crown rots;
stem cankers; insect, grub, or borer injury; drought; compacted or poor
soil; and two other widespread wilt diseases, Verticillium wilt and bacterial
wilt, a common disease in the South. Overall symptoms are the same–a
wilting, withering, and dying of the foliage. Only by close observation
and experience can you determine the true cause.
In general, Fusariium oxysporum attacks plants and is most
severe at air and soil temperatures of 24 C to 32 or 35 C (75 to 90 or
95 F) while the Verticillium wilt fungi (V. Albo-atrum and V. dahliae)
infect plants at somewhat lower temperatures (optimum 21 C or 70 F). Fusarium
wilt is most severe in the southern half of Illinois during warm-to-hot
weather. Transplants that are grown in infested soil usually do not show
typical wilt symptoms until they have been transplanted for some time
and soil temperatures rise to 24 C (75 F) or higher.
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Symptoms
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Typical symptoms of Fusarium wilt include a drooping and yellowing
of the leaves, often starting on one side, and stunting of the plant
(Figure 1). Disease symptoms often commence at the base of the stem
and progress upwards, causing the leaves and flower heads to wilt,
wither, and die. Lower parts of the stem are dark and discolored,
always on the inside and sometimes on the outside (Figures 2 and
3). When infected stems are split, brown to black streaks are evident
in the vascular system. Flowering plants that start from bulbs,
corms, or tuberous roots (crocus, dahlia, freesia, gladiolus, iris,
ixia, lily, narcissus, and tulip) show a dark discoloration within
underground parts (Figures 4 and 5) that commonly extends into the
leaf bases. Seedlings of all flowering plants may suddenly wilt,
collapse, and die.
Masses of white or pinkish Fusarium spores (conidia)
are formed in fungus fruiting bodies, called sporodochia, on the
surface of infected or dead stems, usually near the soil line, or
on infected bulbs and corms.
Disease and symptom development are extremely dependent upon air
and soil temperatures. Symptoms are usually absent or mild at temperatures
below 21 to 24 C (70 to 75 F) and are most severe at constant temperatures
of 29 to 32 C (80 to 90 F). Plants that are grown at low temperatures
may be infected, yet show no symptoms until the temperatures rise.
Specific additional symptoms that occur on selected flowering plants
are given in Table 3.
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Figure
2. Cross sections of carnation stems infected with Fusarium
wilt. Vascular discoloration is dark brown (IL Natural History
Survey photo).
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Figure
3. A black, elongated lesion often develops on one side of
an aster stem infected with Fusarium wilt. Branches on the same
side of the stem usually die suddenly (IL Natural History Survey
photo).
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Disease Cycle
The Fusarium wilt fungi (Figure 7) invade the root systems or other underground
parts of their host plants through wounds that are caused naturally by
the growth of young rootlets through the soil and by wounds in older roots
that are made during transplanting and cultivating; by root-feeding organisms,
such as insects or nematodes (burrowing, lesion, root knot, sheath, sting,
stubby-root, and stunt); and by man-made injuries during transplanting,
cultivation, harvest, sorting, and grading. Once within the plant, the
fungus grows and multiplies in the vascular system (water- and food-conducting
tissues) of the roots. It then moves upward in the plant by spores (macroconidia
and microconidia) that are transported in the sap stream where they become
lodged, germinate, and affect new plant parts; or the fungus extends its
colonization as it grows in the vascular tissue of the host. The normal
flow of liquids and nutrients from the roots to the foliage is greatly
reduced or stopped because the conducting tissue becomes partially plugged
or killed by fungal mycelium and spores, or by the overgrowth of neighboring
cells. Toxic substances are believed to be secreted by interaction of
the fungus and the host plant. These materials apparently cause the wilting
and eventual death of the plant. Wilt symptoms typically are not observed
until the fungus has colonized the underground parts of the plant.
Resting structures (chlamydospores) are formed within infected plant
parts. After the host plant dies or the growing season ends, the Fusarium
fungi survive as mycelia and chlamydospores, overwintering in dead plant
parts; or, the fungi may live in the soil indefinitely in the absence
of the host plant, especially if the soil is warm, as in a greenhouse.
Chlamydospores are stimulated to germinate by exudates from the roots
of a host plant which they then infect.
The Fusarium fungi spread from plantings of contaminated
seed cuttings, transplants, tubers, roots, corms (Figure 4), and bulbs
(Figure 5). In addition, mycelia, conidia, and chlamydospores may be transported
in infested soil by normal tillage operations, in soil clinging to hand
tools, equipment, or shoes, by wind, and by surface drainage water. Once
contact is made with a new plant host, the fungus again invades the underground
parts, progresses upward, and the cycle is repeated. Soil moisture and
soil reaction (pH) have little effect on Fusarium wilts, as the fungi
establish themselves and thrive in a wide range of soil types.
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Control
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The Fusarium wilt fungi are difficult to control. Schemes to eradicate
the pathogen are limited by the ability of the fungi to survive
in soil for long periods, with or without a host plant, and the
colonization of the vascular tissues within a plant.
The first step in managing Fusarium wilt is proper diagnosis. Only
by laboratory culturing of infected plant material can you positively
identify Fusarium as the causal agent. Similar symptoms
are produced by other pathogens. Once Fusarium has
been identified as producing the symptoms expressed in the host,
several measures can be taken to reduce the effects of the disease.
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Figure
4. Fusarium corm rot of gladiolus (IL Natural History Survey
photo).
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Figure
5. Fusarium basal rot of narcissus. Longitudinal section of
an infected bulb, showing how the rot progresses up the scales
from the basal plate.
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- Disinfest greenhouse, seedbed, and potting soil before planting.
Use either steam (hold soil at 82 C [180 F] for 30 minutes at the coolest
spot or 71 C [190 F] for one hour) or fumigate the soil with methyl
bromide, chloropicrin, Vorlex, or Vapam Soil Fumigant. These chemicals
also will control other pathogens, weeds, insects, and nematodes in
the soil. You should also treat containers, benches, work surfaces,
tools, and other equipment. Fumigation is usually done by larger growers
and commercial applicators who are licensed to handle restricted-use
chemicals. The manufacturer's directions should be followed carefully.
- Do not grow susceptible plants in Fusarium-infested soil where the
same or closely related plants have grown previously. A rotation of
5 to 10 years or more may help to reduce the amount of infection.
- Purchase only healthy, top-quality, disease-free seed, cuttings, transplants,
bulbs, corms, or other plant material. Start with culture-indexed cuttings
or clean, mother-block stock plants of carnation, chrysanthemum, and
geranium from a specialist propagator, preferably CVI (culture-virus-indexed)
stock. Fusarium-resistant varieties and cultivars are available for
growing in wilt-infested soil. Varieties and cultivars are listed in
seed or other catalogs for the following garden and greenhouse flowers:
Aster (China-aster), carnation, chrysanthemum, gladiolus, lily, marigold,
narcissus, sunflower, tulip, and zinnia.
- Plant in fertile, well-drained, wilt-free soil. Control soil insects
by following the suggestions of University of Illinois Extension entomologists.
- In home gardens, promptly dig up and burn or haul away with the trash
all infected plants including the roots. Do not bury this debris or
place it in the compost pile.
- Avoid overwatering, deep plant-ing, overfertilizing with nitrogen
or phosphorus, and injuring plants when planting, cultivating, harvesting,
grading, or sorting.
- Where feasible, drench growing plants, such as those in containers
or greenhouse benches, using a fungicide or mixture suggested in Commercial
Growers Landscape and Turfgrass Pest Management Handbook. This circular
is revised annually. Carefully follow the manufacturer's directions
as printed on the package label.
- For Gladiolus, Iris, Lily, Narcissus, and Tulip
Carefully dig bulbs or corms early in dry weather, and dry them
immediately in the shade, using warm-to-hot circulating air. Avoid
sunburning, bruising, or cutting the bulbs and corms when digging,
cleaning, sorting, or grading. Carefully inspect all bulbs and corms
immediately after digging, before storage, and again before planting.
Discard all those showing decay or rot lesions. Fungicide-treated
bulbs and corms (see below) should be planted in clean or fumigated
soil, where the disease has not occurred in the past 5 to 10 years
or more.
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A. For Gladiolus Corms and Cormels
Within 24 hours after digging, soak disease-free corms in a basket
or loosely woven sack for 15 to 30 minutes in warm water (80 to 85 F
or 27 to 29 C) containing one of the fungicides listed in Commercial
Growers Landscape and Turfgrass Pest Management Handbook.
Clean the corms when the old and new corms separate easily, then hold
at the curing temperatures (85 to 95 F or 29 to 32 C) for several days
before storing at 40 to 50 F (3.5 to 10 C). Inspect corms carefully
before planting and discard all those showing rot lesions. All healthy
appearing corms should be dipped in Benomyl, Mertect 160-F, Topsin M.
or Ornalin as outlined in Commercial Growers Landscape and Turfgrass
Pest Management Handbook.
Treat fully dormant cormels in January with hot water to greatly reduce
the number of rot-producing pathogens. Dormancy varies with the gladiolus
variety and the temperature under which the variety was grown and stored.
Cure cormels for one week or longer at 95 F (32 C), and then presoak
them for 2 days in water at room temperature. Discard the cormels that
float; they are diseased. Soak the remainder in 38 percent USP formaldehyde
solution (1 part to 200 parts of water) for 2 to 5 hours, remove the
cormels from the formaldehyde solution, drain well, place them in wire
mesh containers, and immerse in hot water (135 F ± 1/2 F) for
30 minutes. Immediately after treatment, cool the cormels by plunging
them into clean, cold water for 10 to 15 minutes, then dry rapidly in
thin layers in sterilized trays. Place the cormels in cool storage (40
F or 3.5 C) until ready to plant in wilt-free soil.
B. For Iris Bulbs
Within 24 to 48 hours afer digging, soak the clean bulbs for 15 to
30 minutes in warm water containing benomyl (Benlate, 50WP) or thiabendazole
(Mertect 160-F, 60WP), as outlined for gladiolus corms. The iris bulbs
should be dried rapidly in single layers in open trays after treatment.
Store the bulbs in a cool, dry location, preferably where the temperature
is 48 to 50 F (9 to 10 C) and the relative humidity is 60 to 80 percent.
C. For Lily Bulbs
Soak clean, unblemished bulbs within 24 hours after digging and again
just before planting. Dip bulbs for 10 to 15 minutes in a suspension
of fungicide as outlined in Commercial Growers Landscape and Turfgrass
Pest Management Handbook. These fungicides also can be blended into
the soil just before planting or applied as a drench after potting.
Carefully follow the manufacturer's directions as printed on the container
label. Keep stored bulbs cool after treatment until ready to plant.
Propage using only rot-free scales. Dust with thiram, 50-75WP or thiram
plus benomyl, 50WP. Carefully follow label directions. Shake small,
weighed amounts of scales in a paper sack containing the proper amount
of fungicide. Plant at once in steamed or fumigated soil.
D. For Narcissus Bulbs
Soak clean, blemish-free bulbs within 24 hours after digging in a
suspension of fungicide as outlined for gladiolus corms in Commercial
Growers Landscape and Turfgrass Pest Management Handbook. Store dry,
treated bulbs in a cool (55 to 60 F; 13 to 15 C), well-ventilated area
until ready for planting.
E. For Tulip Bulbs
Soak clean, healthy-appearing bulbs within 24 to 48 hours after digging
in a suspension of fungicide as outlined for gladiolus corms in Commercial
Growers Landscape and Turfgrass Pest Management Handbook. Store treated
bulbs in a dry, well-ventilated area below 59 F (15 C) until ready for
planting.
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