Fusarium in Tulips
How to identify Fusarium infection in tulip bulbs and manage its wrath through non-chemical means.
Tulip forcing in 2002 has been difficult. In many
greenhouses, tulip crops were uneven and, overall, very short. Upon flowering,
many pots had one or more stems with blind shoots, where the bud may have been
aborted at a relatively late stage (perhaps when it was 1/2-inch long) or at a
much earlier stage, where only a blackened stump and tiny remnants of a very
small flower were present. In either case, the value of the product was
severely compromised. These kinds of problems were seen in both pot and cut
crops, and in landscape plantings as well. A major culprit in all these
problems is Fusarium, an important disease in many bulb crops, but one that
poses special problems for tulips.
In an earlier article in GPN ("Flower Bulb
Transportation and Handling," August 2001), the relationships of Fusarium
infection, ethylene production and forcing problems were introduced. In this
article, we'll review some of the known information on Fusarium infection in
tulip bulbs and describe some non-chemical remedies for its management.
Recognizing Fusarium infection
The most common Fusarium in tulips is Fusarium oxysporum
Schlecht. f. sp. tulipae, and it can be a problem wherever tulip bulbs are
produced. Tulip bulbs infected with Fusarium are easily recognized because of
the black appearance of infected bulbs. Another easy way to detect Fusarium is
to smell the bulbs. Infected bulbs have a distinct, sour smell as a result of
the fungus degrading the bulbs' tissue. They may also have white mycelium
(mold) growing on the surface, and this is usually concentrated on the basal
part of the bulb. Still other bulbs may be very lightweight as a result of the
fungus consuming the starches and other scale components. Bulbs with a severe
infection might show a somewhat opened bulb tip with the protruding leaves
dried out. Multiple fungi can be present on a tulip infected with Fusarium, for
example, Pennicilium. This fungus is distinguishable from Fusarium as it is
bluish-green. With only superficial growth on the bulb's surface, Pennicilium
is not a major problem.
Field and production factors
Infection of tulip bulbs by Fusarium is more likely during
growing seasons with high soil temperatures from the period of flowering (i.e.,
early May) until digging in late June to mid-July. Thus, Fusarium is
exacerbated in warmer growing seasons. Past research has indicated that later
digging tends to increase Fusarium infection due to the normal increase in soil
temperature in late spring. On the other hand, early harvesting to avoid warm
soil temperatures is not an answer, as bulbs are not properly mature with early
In the case of Dutch production, there are many Á suggestions
as to the sudden increase in Fusarium over the past two seasons, including
changes in farming practices and bulb handling equipment; regulatory changes
affecting fungicide availability; buildup of spore and inoculum in the soil;
and possibly the appearance of one or more "new" Fusarium strains
that could be more resistant to fungicides and/or generally more aggressive in
their infection and spread.
Fusarium, gummosis and ethylene
Aside from direct effects of the fungus on a bulb, a much
larger problem comes from the fact that the Fusarium produces a large quantity
of the plant hormone ethylene. Ethylene can have several negative effects,
including flower abortion, uneven, stunted growth, reduced rooting and gummosis
(external or internal blobs of a clear to brownish-tan substance that
ultimately hardens like peanut brittle, without the peanuts, of course). In
severe cases, the external "gum" can cement numerous bulbs together
into a cluster (more like peanut brittle!). Often, the gummosis is only
produced inside the bulb ("internal gummosis"), filling up the spaces
between the bulb scales. The bulb must be cut open to see internal gummosis.
Another confounding factor in the diagnosis of ethylene
problems is the timing of ethylene exposure. Gummosis is more commonly
expressed in tulips exposed to ethylene shortly after digging, that is, in mid-
to late July. The same cultivars exposed to ethylene late in the season (for
example, after shipment to the United States) will often not develop any gummosis
at all, but may still show 100-percent flower abortion upon forcing.
While most of the symptoms of tulip ethylene exposure are
deleterious (e.g., flower abortion), others are not specifically problematic
(e.g., gummosis). If the flower of a tulip cultivar aborts due to ethylene
exposure, the bulb is obviously worthless. On the other hand, the presence of
some gummosis is not an indication the shipment should be refused. Cultivars
vary in their sensitivity to ethylene and their expression of gummosis symptoms.
For example, certain tulip cultivars may exhibit gummosis upon ethylene
exposure but are somewhat immune to flower abortion from the same ethylene.
Thus, the presence of gummosis in a cultivar in a shipment only indicates that
that cultivar was exposed to ethylene (which probably occurred well before
shipment), but it does not specifically indicate that the entire shipment was
exposed, nor does it specifically mean that the affected bulbs will show
problems upon forcing.
Due to the complex interaction of cultivar, symptom
expression and varying times after digging when these problems can occur, you
should immediately contact your supplier if you receive a shipment with a
substantial proportion of Fusarium or gummosis tulips. Long-standing advice has
been to seriously consider discarding the lot if more than 10 percent of the
bulbs are infected by Fusarium. This is, again, due to injury from the large
quantity of ethylene that can be produced from the infected bulbs. style="mso-spacerun: yes">
What to do?
Since Fusarium-infected bulbs continue to produce ethylene
after planting, such bulbs can injure other bulbs within a pot or a cut flower
forcing crate. Thus the old adage that one bad apple spoils the batch applies
equally well to planted tulips.
During planting operations, bulbs should be inspected, and
those showing any signs of Fusarium infection should be discarded. Also discard
any bulbs that are "light" (having been consumed already by the
fungus) and any with a sour smell (sure evidence of Fusarium actively working
on the bulb). It cannot be emphasized enough how important this step can be to
help with uniformity of the pot or cut flower crate during forcing. The utility
of dipping or drenching with fungicides by U.S. forcers is questionable, as the
injury resulting from ethylene exposure has already mainly occurred.
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Thanks are expressed to the SAF-ARS National Floriculture
Research Initiative and the Dutch Exporters' Association for flowerbulbs and
nursery stock and for financial and material assistance with topics reported