Java fern (Microsorum pteropus) galls/disease

Introduction

Plant diseases are not very common in planted aquariums, but Java fern can be infected by a fungus; Synchytrium cfr. stachydis. The fungus grows inside the plant tissue and causes gall lesions to form on the surface of the leaves and rhizomes. This interferes with the growth of the plant and eventually leads to most surfaces being covered and the plant dying. The galls appear as green protrusions that cause twisting of the leaf surface. They may appear as sparse dots before forming denser clusters over time. This causes deformation of younger leaves and twisted growth. In the long run, the fungus is a slow but sure death sentence for the plant. There is no known cure, which adds to the severity of the disease.

An excerpt from Tri-Ology Vol 58 Number 2 (2019), published by the Florida Department of Agriculture and Consumer Services, describes the Synchytrium fungus as follows:

Identifying the disease

Galls start as rather innocuous small galls. When they are sparsely distributed, some folks mistake them for Java fern spore capsules (pictured below).

The dots in the picture above are not fungal galls, but the usual sporophytes found on the underside of Java fern leaves. Sporophytes are usually brown or black, circular and evenly distributed on the underside of the leaves. This is different from the fungal galls, which can form on either the upper or lower surface of the leaf and rhizome and are green in colour. The galls also cause twisting and deformation of the leaf surface, which is not seen in natural Java fern sporophytes.

As the gall density increases, the java fern plant gets increasingly deformed. The galls feel crunchy to the touch.

Fungus galls smoother this entire young leaf.

Galls causes dents and deformation of the leaf surface.

How it spreads

The disease is spread by contact and by spores in the bile structure. It spreads by contact to neighbouring Java fern. It spreads to different subspecies of Java fern. It spreads slowly, so it can take a long time for adjacent plants to become infected. It does not spread to other plant species.

The spores of the fungus are very resilient, so even after all the infected Java fern plants have been removed from a tank, the tank may still contain spores that can infect newly introduced plants. It can take many weeks for new galls to appear, so confirming whether a tank is 'clean' or not can take a long time. Synchytrium's terrestrial cousins are known to have spores that last for many years, and this has been particularly damaging to aquatic plant farms. Retailers who are unaware of the problem may continue to sell infected plants, and the condition can be difficult for new aquarists to recognise.

In one case, a commercial grower removed all the affected leaves from an infected Java fern and submerged the plant, and the new leaves came out clean. However, when the plants were submerged in water, the fungal galls reappeared after a few months. This suggests that when a plant is infected, the fungus can lie dormant in the rhizome for a long time and resurface when the opportunity arises.

Cure?

There is currently no known cure for this fungus. There is no approved chemical treatment for the land form of this fungus that affects potato plants. The main method of dealing with this fungal disease is exclusion - i.e. not having any infected plants in the first place.

If a tank already contains diseased Java fern plants, the only safe solution is to completely tear them out and start again with new material. Synchytrium fungus spores remain dormant for years. Newly infected plants also take months to show more obvious symptoms, making it extremely difficult to test whether or not a tank is truly free of the disease. Bleaching previously used equipment seems to work, but organic materials such as aquasoil should not be reused.

Hobbyists are advised to carefully inspect any Java fern they purchase to avoid infecting their tanks in the first place. Tissue culture sources are one way around this problem, but tissue culture Java ferns are not commonly sold commercially.