Fusarium in Tulips
Tulip forcing in 2002 has been difficult. In manygreenhouses, 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 beenaborted at a relatively late stage (perhaps when it was 1/2-inch long) or at amuch earlier stage, where only a blackened stump and tiny remnants of a verysmall flower were present. In either case, the value of the product wasseverely compromised. These kinds of problems were seen in both pot and cutcrops, and in landscape plantings as well. A major culprit in all theseproblems is Fusarium, an important disease in many bulb crops, but one thatposes special problems for tulips.
In an earlier article in GPN (“Flower BulbTransportation and Handling,” August 2001), the relationships of Fusariuminfection, ethylene production and forcing problems were introduced. In thisarticle, we’ll review some of the known information on Fusarium infection intulip bulbs and describe some non-chemical remedies for its management.
Recognizing Fusarium infection
The most common Fusarium in tulips is Fusarium oxysporumSchlecht. f. sp. tulipae, and it can be a problem wherever tulip bulbs areproduced. Tulip bulbs infected with Fusarium are easily recognized because ofthe black appearance of infected bulbs. Another easy way to detect Fusarium isto smell the bulbs. Infected bulbs have a distinct, sour smell as a result ofthe fungus degrading the bulbs’ tissue. They may also have white mycelium(mold) growing on the surface, and this is usually concentrated on the basalpart of the bulb. Still other bulbs may be very lightweight as a result of thefungus consuming the starches and other scale components. Bulbs with a severeinfection might show a somewhat opened bulb tip with the protruding leavesdried out. Multiple fungi can be present on a tulip infected with Fusarium, forexample, Pennicilium. This fungus is distinguishable from Fusarium as it isbluish-green. With only superficial growth on the bulb’s surface, Penniciliumis not a major problem.
Field and production factors
Infection of tulip bulbs by Fusarium is more likely duringgrowing seasons with high soil temperatures from the period of flowering (i.e.,early May) until digging in late June to mid-July. Thus, Fusarium isexacerbated in warmer growing seasons. Past research has indicated that laterdigging tends to increase Fusarium infection due to the normal increase in soiltemperature in late spring. On the other hand, early harvesting to avoid warmsoil temperatures is not an answer, as bulbs are not properly mature with earlydigging.
In the case of Dutch production, there are many Á suggestionsas to the sudden increase in Fusarium over the past two seasons, includingchanges in farming practices and bulb handling equipment; regulatory changesaffecting fungicide availability; buildup of spore and inoculum in the soil;and possibly the appearance of one or more “new” Fusarium strainsthat could be more resistant to fungicides and/or generally more aggressive intheir infection and spread.
Fusarium, gummosis and ethylene
Aside from direct effects of the fungus on a bulb, a muchlarger problem comes from the fact that the Fusarium produces a large quantityof 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 thatultimately hardens like peanut brittle, without the peanuts, of course). Insevere cases, the external “gum” can cement numerous bulbs togetherinto a cluster (more like peanut brittle!). Often, the gummosis is onlyproduced inside the bulb (“internal gummosis”), filling up the spacesbetween the bulb scales. The bulb must be cut open to see internal gummosis.
Another confounding factor in the diagnosis of ethyleneproblems is the timing of ethylene exposure. Gummosis is more commonlyexpressed 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 (forexample, after shipment to the United States) will often not develop any gummosisat all, but may still show 100-percent flower abortion upon forcing.
While most of the symptoms of tulip ethylene exposure aredeleterious (e.g., flower abortion), others are not specifically problematic(e.g., gummosis). If the flower of a tulip cultivar aborts due to ethyleneexposure, the bulb is obviously worthless. On the other hand, the presence ofsome gummosis is not an indication the shipment should be refused. Cultivarsvary in their sensitivity to ethylene and their expression of gummosis symptoms.For example, certain tulip cultivars may exhibit gummosis upon ethyleneexposure but are somewhat immune to flower abortion from the same ethylene.Thus, the presence of gummosis in a cultivar in a shipment only indicates thatthat cultivar was exposed to ethylene (which probably occurred well beforeshipment), but it does not specifically indicate that the entire shipment wasexposed, nor does it specifically mean that the affected bulbs will showproblems upon forcing.
Due to the complex interaction of cultivar, symptomexpression and varying times after digging when these problems can occur, youshould immediately contact your supplier if you receive a shipment with asubstantial proportion of Fusarium or gummosis tulips. Long-standing advice hasbeen to seriously consider discarding the lot if more than 10 percent of thebulbs are infected by Fusarium. This is, again, due to injury from the largequantity of ethylene that can be produced from the infected bulbs.
What to do?
Since Fusarium-infected bulbs continue to produce ethyleneafter planting, such bulbs can injure other bulbs within a pot or a cut flowerforcing crate. Thus the old adage that one bad apple spoils the batch appliesequally well to planted tulips.
During planting operations, bulbs should be inspected, andthose showing any signs of Fusarium infection should be discarded. Also discardany bulbs that are “light” (having been consumed already by thefungus) and any with a sour smell (sure evidence of Fusarium actively workingon the bulb). It cannot be emphasized enough how important this step can be tohelp with uniformity of the pot or cut flower crate during forcing. The utilityof dipping or drenching with fungicides by U.S. forcers is questionable, as theinjury resulting from ethylene exposure has already mainly occurred.
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Thanks are expressed to the SAF-ARS National FloricultureResearch Initiative and the Dutch Exporters’ Association for flowerbulbs andnursery stock and for financial and material assistance with topics reportedherein.