• Cutting out Botrytis lesions the day before the crop is dropped and deleafed is likely to make a significant contribution to reducing disease.

• Keeping the relative humidity of the greenhouse down will reduce Botrytis infection of flowers, but not stems.

These are two of the conclusions drawn from three years of study of the disease in Auckland. Our research has also led to the development of some biological controls which are very effective in controlling stem infections. These biologicals outperformed four available fungicides under experimental conditions.

"Botrytis can be savage"
Any tomato grower will tell you that if Botrytis gets away in a crop of greenhouse tomatoes, it can be quite savage. It reduces yield and kills plants. Once Botrytis is established, it is difficult to bring under control. The sprays currently available do not do a one hundred percent job, and they cannot be expected to. This is because it is difficult to get good coverage (the intention of canopy sprays) and because canopy sprays do not target the infection sites. Also, the way we manage the plants can give Botrytis just the break it needs. Photo 1.

"being a friend to Botrytis"
If there are a lot of Botrytis lesions in a crop, and you take off the leaves and drop the crop at the same time as cleaning up the infections, then you are being the best possible friend to Botrytis. The new wounds made by leaf and lateral removal are quickly infected by spores from the old infections. How does this happen?

When a tomato plant with stem, leaf, or flower lesions is deleafed and dropped, spores are shaken from the lesions in their hundreds of thousands, even millions. They are borne on air currents in the greenhouse and settle back on the plants, including the stem wounds freshly made by deleafing. Within a week, there can be new Botrytis lesions in some of the stem wounds up to 1 or 2 cm long. As a result, the disease becomes worse in succeeding weeks.

"spores settle quickly"
The plants have to be dropped (for a long crop) and leaves removed. But it is possible to get the leaves off without giving Botrytis such a good opportunity. We have monitored the levels of Botrytis spores under a number of conditions and spores were found to settle out of the air quite quickly. In an undisturbed house, spore levels are about 0-30 spores per cubic metre. Immediately after a house has been disturbed by dropping and deleafing, levels soar to hundreds or even thousands per cubic metre. However, within 2-3 hours this level has dropped to a few tens or less than 100, and by early morning the next day, you would never know that the house had been disturbed: spore concentration in the air is back to 0-30 per cubic metre.

Figure 1. Aerial Botrytis spore concentration influenced by cultural operations.

An option to minimise new infections is to work your way through the crop removing lesions and infected debris the day before dropping. This allows time for any spores released into the air to settle out and land on intact parts of the plant which are normally not susceptible.

When leaves with a heavy deposition of spores on them are disturbed, some of the spores will be released back into the air. But the numbers are very small compared with the numbers released from disturbed sporulating lesions - only about 1-3%.

"high humidity allows flower infection"
Botrytis proliferates best in moist conditions. If the relative humidity of the greenhouse rises to near 100% during the night and stays there for any length of time, Botrytis will infect the flowers. But this only occurs, of course, if Botrytis spores are present on the flowers. We have carried out extensive testing of the effects of relative humidity on flower infection under controlled conditions to define just how dependant infection is on relative humidity. Our conclusion is that above 85% RH, flower infection rises rapidly with increasing RH. Below 80%, infection can still occur, but only at a low level. On the other hand, humidity has only a very small effect on stem wound infection.

"warm temperatures increase flower infection but reduce stem infection"
We all know that with the coming of warmer conditions in spring, Botrytis becomes less of a problem. This is backed up with our findings for stem infection. But flower infection actually increases in the warmer weather. More flowers abort and drop off, reducing yield potential. This is not as serious as it sounds, because the flower lesions become far less aggressive - the Botrytis does not reach back into the truss, as most infected flowers just drop off. The plant is also able to produce more flowers to compensate in the warmer weather.

"stemming infection with biologicals"
Knowing that stem wounds are the main problem caused by Botrytis, we are developing a system of applying other fungi which do not cause disease in tomatoes to stem wounds. What we have found is that quite a number of fungi are antagonistic to Botrytis, and will prevent it infecting a fresh wound. The best of them work better than fungicides.

We have collected fungi from commercial greenhouse tomato crops and tested them for effectiveness against Botrytis. Two of the most effective types are among the most common fungi in the greenhouse: Trichoderma and Cladosporium. Cladosporium is also one of the fungi most often found in the air of houses, factories, etc.

The Cladosporiums are not the same as the fungus causing leaf mould of tomatoes. The leaf mould fungus used to be called Cladosporium fulvum. As a result of more recent knowledge, however, it has been renamed Fulvia fulva. Trichodermas and Cladosporiums of the type we have isolated and tested do not cause any disease in tomatoes under normal conditions.

We have tested these fungi on 3-4 month old plants and have achieved high levels of control. They have been found as effective on terminal wounds (these are highly susceptible, often being 90 - 100% infected) as deleafing wounds (usually being only 25% infected). Even when poor coverage of the wounds was simulated, control was still 100% effective. Photo 2.

Figure 2.
Cladosporium control of Botrytis infection of terminal and leaf removal wounds of tomato plants.

Wound type	Cladosporium percentage	Infection
terminal	not applied	81.0
leaf removal	not applied	27.0
terminal	applied 0%
leaf removal	applied 0%

"targeted application"
We see the most effective way to use these biologicals is to apply them to the wounds as they are made. We have tested 2 clipper-application devices which do this and both give a high level of control.

Photo 3.
Gelsecateur applied biologicals suppressing Botrytis infection of terminal wounds of tomato plants.

Photo 4.
Gel and spray clippers, used to apply biologicals via the cutting blade.

Early indications are that clipper-application of biologicals would only be needed in the first 2-3 months of a crop's life, and would render canopy sprays for Botrytis unnecessary during this period. It will take longer to deleaf with clippers. But once workers are familiar with the devices, the labour cost will come down.

Figure 3.
Gelsecateur applied biologicals suppressing Botrytis infection of terminal wounds of tomato plants.

Isolate	Percentage	Infection
no biological control	100.0
Trichoderma 1		33.3
Trichoderma 2	50.0
Cladosporium 1		0.0
Cladosporium 2		0.0
Cladosporium 3		0.0

"future research"
Our research of the last three years has focused on defining the factors which influence infection (spore load, temperature, and humidity), developing the biological control for stem infection and exploring application technology. In the coming three years we hope to trial and make commercially available these biologicals. In addition, we hope to quantify the effects of specific greenhouse environmental control strategies and modified cultural practices on the epidemic development of Botrytis. The aim of this is to produce a set of cultural and environmental control strategies for control of Botrytis which growers can easily implement in their crops.

Acknowledgements:
The experience, advice, assistance, and willing co-operation of a number of growers who allowed us to monitor disease in their crops over the last few years is gratefully acknowledged. It gave us the basis to develop controlled experimental work which would have direct applicability to the field situation.

Source:
The Commercial Grower, March 1996, Vol: 51, Number: (2)