Rotate Crops in the Garden
Three- to four-year rotations are best if you have enough room in
the garden. It is also important to stake or cage tomato plants
because that allows increased air circulation around them and it
makes them easier to spray if you choose to use fungicides.
Tomato Early Blight
There are a number of “blights” that have impacted tomatoes in our
area this year, including early blight, septoria leaf spot and late
blight. Early blight is caused by the fungus Alternaria solani and
is characterized by leaf spots with a bulls-eye appearance. Spotted
leaves yellow and die prematurely, which leads to early defoliation.
This reduces yields, and leaves fruits open to sunscald. The fungus
overwinters on plant residue and can persist for at least a year. It
is also borne on seeds, and can be introduced to the garden on
infected transplants. Early blight occurs under a variety of weather
conditions, but is favored by periods of leaf wetness from dew and
Early blight usually starts on the oldest, lowest leaves
when spring rain hits the ground and splashes overwintered spores
onto the leaves closest to the ground. Good garden sanitation can
help remove much of the fungus, and mulching transplants can reduce
splashing. Clean oat straw, composted grass clippings (not from
herbicide-treated lawns), shredded leaves and red or black plastic
are suitable mulches in the vegetable garden.
work best when they are used to protect plants from infection,
rather than after the fact. Start making applications about two
weeks after setting transplants into the garden. Fungicides labeled
to control early blight include chlorothalonil (Ortho Vegetable
Disease Control), maneb (Dithane), copper-based fungicides (Bonide
Copper), and Bacillus subtilis (Seranade).
Septoia Leaf Spot
Septoia leaf spot is caused by the fungus Septoria
lycopersici and is characterized by numerous small, circular leaf
spots. They are much smaller than early blight leaf spots, and lack
the bulls-eye appearance. Infected leaves yellow and fall
prematurely, resulting in reduced yields and fruits damaged by
sunscald. Like early blight, septoria overwinters on infected plant
debris and is splashed onto the lower leaves of new transplants by
rain or overhead irrigation. Septoria is favored by wet weather and
dew, but is not usually a problem until plants begin to set fruit.
Garden sanitation, crop rotation, mulching and fungicide
applications all help reduce the severity of this disease. The same
fungicides labeled for early blight are also labeled to control
septoria leaf spot.
Tomato Late Blight
Late blight is caused by the oomycete Phytophthora
infestans, and is one of the most destructive tomato (and potato)
diseases. Leaf spots caused by late blight are irregularly shaped,
and appear greasy or water-soaked at first. The spots enlarge
rapidly during wet weather, merging together to kill entire leaves.
You may also see masses of white spores on the underside of affected
leaves under humid conditions. The causal organism can only
overwinter on potato tubers in the northern United States – those
overlooked during harvest or in cull piles on commercial potato
farms. Otherwise, it overwinters in the southern United States and
the spores are blown north on storm currents. Cool, wet weather
favors late blight development. If you grow potatoes, avoid planting
tomatoes or potatoes in those areas more than once every three or
four years. While garden sanitation and staking to improve air
circulation are always good practices, fungicide applications are
the most important control for late blight. The same fungicides
labeled to control early blight and septoria leaf spot are also
labeled to control late blight.
Tomatoes with TMV
Tomato Growing 101
Tomatoes with Late blight