Late blight on tomato

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Image below shows initial symptoms of late blight on tomato in a garden. There is an affected leaflet below the bottom trellis line. The primary symptoms are stem lesions between the second and top lines. See close-up images of those lesions in following images.

Next two images: These dark brown lesions on stems, with white fungal growth developing under moist conditions, are characteristic of late blight. The fungal growth on the stems and also on affected leaves consist of thousands of microscopic spores. The spores can be dispersed by wind to healthy plant tissue, infect the healthy tissue if it is wet for a few hours, and within a few days produce a new lesion.

The disease can be very explosive due to the quantity of spores produced and the quick disease cycle under cool, wet conditions. And it is destructive as diseased tissue is killed quickly. Established lesions cannot be controlled, even with the most effective systemic fungicides that can be used by farmers with a pesticide license.

Leaf lesions due to late blight

Leaf lesions due to late blight

Leaf lesions due to late blight

Fuzzy growth on the underside of leaf lesions (below) is produced by the pathogen under moist conditions and consists mostly of spores.

Lesions (below) with growth and spores of the pathogen that developed while it was in a plastic bag over night.

A single leaflet with a lesion is the first sign of late blight development in this garden.

Close up of the leaf above.

More close-ups of leaf lesions. Click each image for larger views

Below: Plants with numerous initial symptoms of late blight. These plants are too severely affected to be saved. (Photo courtesy Dr. Steve Johnson, University of Maine Cooperative Extension.)

Early stem lesion (below).

Stem lesion with sporulation of the pathogen. (below).

The brown spots on this tomato stem (below) are probably not due to late blight.

Young plants can be infected before transplanting, showing stem lesions (below) ...

Green fruit affected by late blight (below).

Green fruit affected by late blight (below).

Green fruit affected by late blight. Note fuzzy pathogen growth and spores, (below).

Symptoms of late blight on ripe fruit (below).

Below: Symptoms of late blight on ripe fruit (Photo courtesy Dr. Steve Johnson, University of Maine Cooperative Extension.)

Plant killed by late blight (below).

A late season infection

These images (below and the following thumbnails) were taken on 31 August 2009 when late blight was first observed on this farm. These plants only had symptoms of Septoria leaf spot on a few lower leaves 3 days earlier. Last fungicide application was copper 10 days earlier. These plants are too severely affected to be salvageable. Fruit were just starting to ripen in this late planting thus this is a total loss. With the exception of a lengthy rainstorm that started less than 3 days before these symptoms were seen (which is too short a time period for symptoms to develop after infection), weather conditions, being hot and dry during August in this area, did not seem to be favorable for late blight. There was only 2.01 inches of rain, with 1.08 inches falling over 28-30 August. Prior to this event, rainfall was 0.45, 0.07, 0.04, 0.09, 0.18, and 0.01 inches on 1, 2, 10, 13, 22, and 27 August. Average monthly high temperature was 83 F and low was 68 F. This outbreak illustrates that late blight does not occur only when it is rainy and cool. High relative humidity is recognized as providing a sufficient amount of moisture for the late blight pathogen to be able to infect.

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Some weeds are susceptible to late blight

There are several types of nightshade, which are all in the same botanical family (Solanaceae) as tomato and potato, thus it is not surprising that these weeds can also be affected by some of the same diseases. The large, brown leaf spots on the bittersweet nightshade in the images below are symptoms of late blight. Limited sporulation of the pathogen was observed with a microscope on the underside of the spots. This weed was found next to a garden where tomatoes had late blight in 2009. Some leaves are notched, which is characteristic for bittersweet nightshade, as is the vining growth habit. It is a common weed in hedges and landscapes, but not in agricultural areas. It is difficult to eradicate because vines break easily thus whole plants cannot easily be pulled up.

Late blight on bittersweet nightshade.

Upper surface of bittersweet nightshade leaf with late blight.

Lower surface of bittersweet nightshade leaf with late blight.

Flowers and fruit of bittersweet nightshade (below) resemble those of tomato. These are characteristics of the Solanaceous Family.

Close up of flowers and fruit of bittersweet nightshade (below).

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