In The Greenhouse: Tomato Updates

Occasionally, I still get calls about the disease leaf mold, also known as Fulvia fulva, the Latin name. This is one of those problems that is nearly extinct in greenhouse culture today because of all the newer greenhouse varieties that have full resistance to this disease. I remember 15 to 20 years ago when it was one of the most common and devastating diseases in greenhouses.

 

Back then, growers would call about large yellow spots on the upper surfaces of lower leaves, which would quickly travel up the plant and get much worse. The undersides of these spots appear as olive green, velvety growth. Soon, the leaves turn yellow-brown and drop prematurely. Defoliation further up the plant soon follows. This disease thrives in high humidity, i.e. over 90%, and greenhouses are excellent at providing the perfect conditions for its development with all of the irrigation and transpiring leaf surfaces.

 

To avoid this disease in the future, look for a greenhouse variety with full resistance, indicated by a C5 in the disease resistance code. This would include many of the newer types. A C2 variety has partial resistance, so I would not recommend it.

 

If you still get this disease, it is almost certain that you are using an old greenhouse variety, or more likely a field or garden variety of tomato. I do understand that in some markets, heirloom or other garden varieties are in demand and therefore reward the grower with higher sales prices. If this is the case, maneb, manex, mancozeb, or dithane should help.

There is a small but slowly growing contingent of organic greenhouse tomato growers in the U.S. While organic production in the greenhouse is certainly more difficult and challenging for the grower, in some areas the price premium makes it a good business decision.
Unfortunately, in some parts of the country, the high humidity and mild winters make it nearly impossible. Mild winters across the southern states do not provide long enough periods of freezing temperatures to kill lots of the insect pests of greenhouse crops, so they easily survive outside, ready for reinfestation of greenhouses.

 

High humidity across the northeastern, mid-Atlantic, and southeastern states causes diseases to proliferate and sometimes get out of control. The Gulf states get what is known as the double whammy — mild winters and high humidity. Organic growers in this region are few and far between.

 

But, for those growers in any region who would like to try organic practices, the question always comes up as to where they can find good information. The natural assumption about organic questions would be trying to determine which pest and disease management products are acceptable. While this is true, one of the biggest questions about organics is what kind of fertilizer can be used.
For those greenhouse growers using soil culture, any organic fertilizers usable outside can be used inside. However, for growers using any type of hydroponic (soilless) culture, soluble fertilizers are required so that they stay in solution from stock tank to emitter delivery point. Otherwise, clogged drip lines and emitters prevent water delivery to plants. This compounds the problem.

 

One good source is ATTRA, the National Sustainable Agriculture Information Service. The online publication, “Organic Greenhouse Tomato Production” (http://attra.ncat.org/attra-pub/ghtomato.html), is a good place to start. In addition to addressing all of the cultural and nutritional information, it has a long list of resources for more good information at the bottom.

Wouldn’t it be great if you could accurately estimate your tomato yield several weeks before each cluster ripened? This would greatly facilitate marketing and sales. In studying various environmental variables and their effects on yield, a researcher in Japan has come up with a strongly positive correlation between yield and one factor: the amount of solar radiation (light) during the couple of weeks just before anthesis (fully opened flowers). Tadahisa Higashide, working at the National Agricultural Research Center for Western Region of the National Agricultural and Food Research Organization in Zentsuji, Kagawa, Japan, found that total fruit yield and fruit number could be predicted by the amount of solar radiation from four to 10 days just before anthesis. The correlations with light were stronger than those of air temperature during the period before anthesis.