Tomato disease
management requires an integrated approach in both conventional and
organic systems. Many of the same principles and tactics can be used
in both systems, with the most obvious difference being the prohibition
of synthetic fungicide use in organic tomatoes.
Tomatoes are susceptible to many diseases, and in regions with hot,
humid, rainy growing seasons such as the Midwest and parts of the Northeast,
tomatoes are rarely disease-free in the absence of fungicides. However,
tomatoes are also highly resilient, and can tolerate some level of disease.
In conventional and organic systems, the challenge is to find the proper
balance of tactics that assure economically acceptable yield and quality.
Five broad categories of disease management tactics can be considered:
(1) site selection, (2) cultural controls, (3) variety selection, (4)
biological control, and (5) chemical control.
Site Selection
Site selection is both important and problematic. In areas where population
pressures and other factors reduce the amount of land suitable for vegetable
production, there might not be a great deal of choice of field sites.
To optimize disease management, however, fields should be well drained,
have good air movement and have some distance from fields with related
crops. For example, tomatoes and potatoes are both susceptible to late
blight and it is preferable to locate tomatoes as far from potatoes
as possible. This is more important for organic than conventional growers,
since the latter have access to fungicides that do a good job of controlling
most foliar diseases caused by late blight and other airborne fungal
pathogens. Good drainage is critical for managing soil-borne pathogens,
especially Phytophthora and Phythium spp. causing fruit rots, and bacterial
pathogens that spread easily from plant to plant in flooded fields and
are very difficult to control once established. Good air movement results
in improved drying of the plant canopy, resulting in shorter periods
of time in which bacteria and fungi can infect plants.
Cultural Management
This is a broad category of management tactics that includes numerous
common-sense approaches that affect not only disease incidence and severity,
but also plant growth and management of other pests, in both the greenhouse
and field.
The first consideration is to always use clean seeds, as free of pathogens
as possible. For tomatoes, seeds should be tested for the bacterial
pathogens causing bacterial canker, bacterial spot and bacterial speck.
Certain fungi may also be seed borne, but testing is not routinely carried
out for these pathogens.
We recommend hot water treatment for all seed lots testing positive
for a bacterial pathogen, and all untested seed lots. Hot water treatment
is preferred because the bacterial canker pathogen survives inside the
seed coat and is not completely eliminated by surface disinfestations
using Clorox, acid, or other treatments. It is also permitted for organic
tomato production.
Tomato seeds should be placed in a loosely woven cotton (e.g. cheesecloth)
bag, not more than half full, and pre-warmed for 10 minutes in 100°F
water bath for 25 minutes exactly. Old or poor quality seed may be damaged
by this treatment. For more instructions, see Ohio State University
Extension Bulletin 672, “Ohio Vegetable Crop Production Guide
(seed treatments),” also available online: http://ohioline.osu.edu/b672/index.html.
Tomato transplants should be produced in a greenhouse with good sanitation
practices, in pathogen-free soil or planting mix. Environmental conditions
in the greenhouse should be set to minimize periods of high relative
humidity, and plants not be overwatered or handled often.
In the field, crop rotation should be practiced, with at least two years
out of Solanaceous plants in conventional systems. Organic systems require
at least a four-year rotation. Good sanitation is also important in
the field, and for small farms, removal of diseased plant material can
be beneficial and practical. Mulches are generally used to prevent weeds,
but are also helpful in reducing splash dispersal of a number of pathogens.
Plastic mulches are permitted in organic systems but plant-based mulches
are preferred.
Variety Selection
Choice of variety is more often driven by market demands than by disease
concerns. However, varieties with resistance to diseases should be used
whenever feasible, especially in organic systems. Many commercial tomato
varieties are resistant to Fusarium and Verticillium wilt, and my also
be resistant, partially resistant or tolerant to gray leaf spot (Stemphylium),
Alternaria stem canker, bacterial speck, tomato mosaic virus, or root-knot
nematode. Some new varieties have resistance to tomato spotted wilt
virus and bacterial leaf spot (races 1 and 3). Genetically engineered
crops are prohibited in organic systems.
Biological Control
There are few successful examples of biocontrol agents used as external
inputs to control tomato diseases. However, biological control can be
part of the dynamic of the host/pathogen/environment when soils have
been properly managed.
A key component of organic systems, which would also benefit conventional
tomato production, is increasing the organic matter content of soils.
This is generally accomplished through the application of raw or composted
manures. We have found that although tomato varieties respond differently
to compost amendments, there can be a significant yield response in
compost-amended fields. In addition, anthracnose and bacterial spot
were less prevalent when compost was used. Organic matter also tends
to improve several soil characteristics, including tilth, water and
nutrient-holding capacity, and microbial activity.
There are several biological products now on the market to control diseases,
and, in our experience, they can be helpful in managing certain diseases.
However, we have not yet worked with biological control agents that
consistently reduce the incidence or severity of tomato diseases. There
is some promise for Bacillus and related
species, some Pseudomonas spp. and certain fungi, particularly Trichoderma
spp.
Chemical Control
There are a number of synthetic fungicides available for managing the
foliar and fruit diseases that occur regularly or sporadically on tomatoes
in the Midwest and Northeast. These include early blight, late blight,
Septoria leaf spot, buckeye rot, anthracnose and powdery mildew.
The recent introduction of the low risk, broad-spectrum strobilurin
fungicides has increased the options available to the conventional grower.
While the specific recommendations vary in different regions, most involve
alternation of a strobilurin fungicide with another fungicide with a
different mode of action. These fungicides are not effective in controlling
bacterial diseases, so copper applications are also included.
The TOM-CAST disease prediction system is available to assist the grower
in timing of fungicide applications, and reduces the number of fungicides
needed in most years.
Organic growers also have several fungicides available, although none
are synthetic and most must be applied at high rates. Fungicides are
restricted-use materials, and should be applied only when necessary.
These include sulfur compounds, which are quite effective against powdery
mildew, and fixed coppers, which are relatively poor fungicides but
may reduce bacterial disease incidence and severity.
Whether tomatoes are produced conventionally or in an organic system,
diseases can cause significant yield and quality losses if not managed
properly. In both cases, an integrated approach is necessary for optimal
results. While the specific tools vary between the systems, the fundamental
concepts are the same and by taking an integrated approach, the best
possible results can be expected.
