Silly Name, Serious Threat
Sad to say, the drum beat is getting louder. A few years ago, we thought of Black Goo or Phaeomoniella, as a rare new disease affecting young vines in parts of California. There was very little understanding about its origin. We now call it Petri’s Disease in recognition that it’s not new at all, and it is not unique to California. Lionello Petri of Sicily was the first to publish the symptoms in 1912, and bears the dubious honor of having the disease named for him today.
It’s found in vineyards all over the world, including Italy, France, South Africa and Australia to name a few. The disease stunts the vine growth, usually resulting in the affected vines having to be replanted. It’s a costly problem when it occurs.
Real progress in identifying the source and some possible solutions has begun because viticultural experts from all around the world have put their heads together to try to understand why some vines display the characteristic leaf discoloration, emit what looks like black “goo”, and go into rapid decline. Once the global communication began, we realized that we used different names for the same disease, whether Die-back, Esca, Phaemoniella or Black Goo.
Whatever the name, it’s been identified as a fungus that usually comes with the nursery stock. A study funded by the American Vineyard Foundation in 1998 reported that more than 60% of a sample of California grown root stock1 was infected. The percentages may be smaller now, due to increased awareness, but it means that the infection is already present in some of the rootstock on planting day. The infection travels from the rootstock up the trunk and into the shoots. Whether or not it develops from vine to vine within the planting seems to depend on a variety of factors including luck.
This was painstakingly researched, but still generated denial and anger among wholesale nurseries. They insisted that vine decline had to do with stress caused by lack of water or that the issue is in the soil. Those theories are still out there, but as of this writing in February, 2006, the evidence points to the plant material.
The problem originates with infected “rootstock mother vines”2 in wine regions all over the world. Grafting techniques can exacerbate the problem, as can stress, but the root problem is the mother rootstock. Spores can travel from wound to wound at pruning time, so fungicide on the cuts may reduce the problem, but it doesn’t address the core issue.
There is some brightness in all this gloom. Awareness will certainly lead to competent methods of identifying infection and treating it before the plant material gets to the grower. Already we’ve learned that if the nursery gives the rootstock a hot water treatment3 before shipment, it will significantly reduce the incidence of Petri’s Disease (and other pathogens), but does not eliminate it. Growers, now aware of the problem, can do a vine-by-vine, labor-intensive inspection for signs of Petri’s Disease4 before planting, if they have the resources to do it.
So far here in the Napa Valley, Petri’s Disease is a small issue compared to some others, and we are fortunate that a dedicated handful of scholars from around the world have taken on the challenge to identify it and search for solutions. Their work will likely arm us with the knowledge and tools we need should the threat become major.
- Rootstock: Wine grapes, called vitis vinifera, cannot be grown on their own roots, due to lack of resistance to certain soil pests. They are grafted onto various rootstock hybrids that are resistant to the pests. Additionally, the hybrids are chosen for other beneficial traits, such as low or high vigor, drought resistance, etc.
- Mother vines: The rootstock (see 1 above) comes from state certified “rootstock mother blocks” (sections of vineyards) in most viticultural regions around the world. The mother blocks are the source of the vast amount of plant material that goes out to wholesale nurseries for propagation and grafting on site.
- Hot water treatment: Submerge the rootstock in 122 degrees F. water for 1/2 hour, followed by cool water for an hour.
- Checking for signs of Petri’s Disease: 1. Look for poor callusing at the graft 2. Bend the rootstock to see if the graft breaks 3. Buy a small number of vines to check the percentage of problems by doing the above, or send them out for professional inspection before ordering large quantities.