Penicillium Stem Rot of Greenhouse Cucumber

Introduction Penicillium stem rot was first reported in Ontario and subsequently in Europe. It is a major disease in Ontario, particularly on rockwool grown crops1. The fungus which causes the disease, Penicillium oxalicum, is common in soil and on decaying organic matter in B.C. but not all isolates of the fungus cause disease. In September 1994, a virulent form of the fungus was found to be causing stem rot in one B.C. greenhouse.

Description of Penicillium Stem Rot

Infection appears initially as water-soaked, translucent areas at the nodes, especially nodes that have been pruned. (Do not confuse this with sunscald). Within a day or so, this becomes a pale-brown canker with a blue-gray to blue-green fungal growth on the surface giving off a cloud of spores. The stem splits open easily and masses of fungal growth and spores can be found inside. The cankers expand to a few centimetres above and below the node. They have dry, pale-brown edges. When not producing spores, they look similar to Botrytis and Gummy Stem Blight cankers. Stems infected with Penicillium collapse faster than stems infected with these other diseases. Girdling at the infected node may occur within 4 to 5 days from infection, and the top of the plant wilts and dies.

One of the most noticeable early symptoms are the dried-up, tan-coloured leaves associated with the infected stems. Other leaf symptoms include puckered leaves where infected veins on the underside of the leaf become brown and dried. These usually occur near a stem lesion.

During fruit production, the fungus can also be found sporulating on senescent flowers. This develops into a soft, brown rot which extends back into the fruit.

Small wounds on the skin of healthy fruit can also become infected during harvest. The fruit rot then develops during storage, often accompanied by bacterial soft rot.

As leaves naturally senesce they can become infected, resulting in large brown lesions which become covered with spores under humid conditions

Environmental Conditions Favouring Disease

The fungus produces abundant blue-grey to blue-green spores on the surface of cankers and in the stem underneath. Spores are spread mostly in air, but can also be spread on pruning tools and hands.

The spores infect pruning cuts and wounds on fruit, especially under conditions of high humidity, when sap exudes from the cuts. In Ontario, the disease is more prevalent on outside rows. It is associated with excessive nitrogen fertilizer, which produces a "soft" crop, and the stress of too many stem fruits. It is more severe on crops grown in rockwool than in soil. No differences in susceptibility between cultivars have been reported.

The fungus survives between crops, probably on plant debris, weeds and soil.

Disease Management

Because spores are produced abundantly and are easily spread, the disease can be difficult to control. Avoid overhead dripping water. Greenhouse temperatures should reach the daytime temperature at least one hour before sunrise to minimize dew formation on the plants.

Avoid using high nitrogen fertilizers such as urea or ammonium nitrate.

If the disease has been found in the greenhouse, cut all infected plants below the cankers and carefully bag and remove them from the house to prevent spread of the spores. Bury or burn infected plants. Do not leave them on nearby cull piles.

Remove all stem fruits to height of about 1 metre. Cut side shoots with a sharp knife that is disinfected at intervals during pruning. Use a 5% solution of 12% strength bleach for a pruning dip. Other treatments such as a 1.5% solution of Lysol brand disinfectant or 70% ethanol (shellac thinner) require a 1 minute soak to kill the fungus. Or, soak in a 1% solution of Virkon for 5 minutes. Disinfect pruners frequently when working in disease "hot spots". Work in diseased areas last if possible.

At harvest, cut fruit with a knife instead of pulling. Leave a short stem, about 5 mm. Handle fruit carefully to prevent wounding and cool as quickly as possible. Ensure that the storage area is well ventilated.

Chemical Control

Rovral has been shown to be effective, although some Penicillium isolates have become insensitive to this product. It may be practical to use spot treatments immediately after removing infected material. This will protect any wounded tissue exposed to the spores.

Identification

Penicillium fungi are frequent invaders of dead and dying tissue from other causes. If you suspect that you may have Penicillium stem rot, contact your crop advisor and submit a sample for testing to the BCMAL Plant Diagnostic Laboratory.

¹Menzies, J.G. and Jarvis, W.R. (1994) Penicillium Stem Rot. Sec. 22.14 Diseases and Pests of Vegetable Crops in Canada, Can. Phytopath. Soc. and Ent. Soc. Can.

Pest Management Note 95-04
Updated June 2006

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