Getting to the Root of the Matter: Soilborne Diseases of Tomato

It is often easy to tell if a plant is sick when it is covered in spots or leaves and fruits are rotting away, but it can be much more difficult to tell when a plant has an infection in its roots or stems. These lower portions of the plant are essential for health and productivity, but are often attacked by pathogens that reside in the soil. Below are some of the most important soilborne diseases of tomatoes and methods for managing them.

How to identify soilborne diseases of tomato

V-shaped necrosis and yellowing of leaves characteristic of Verticillium wilt.

In general, some clues that indicate if a tomato has a soilborne disease includes stunting, wilting, yellowing, dieback and reduced yield. These are also symptoms of nutrient deficiencies and some viruses, so it is important to rule out these possibilities.

Verticillium wilt: Distinctive V-shaped lesions form on the edges of leaves, with V-shaped dead tissue surrounded by a yellow halo. Plants wilt and have yellowing and dieback. Plants may wilt during the day and recover overnight. The inside of the stem has brown discoloration.

Tomato plant with Fusarium wilt. Note the yellow discoloration on only half of the plant.

Fusarium wilt: Plants have yellowing, dieback, and wilting. Sometimes only half a leaf or leaves on only one half of the plant turn yellow and die. The inside of the stem has brown discoloration near the soil line and discoloration may continue up the stem. Roots may look brown and rotten.

Corky root rot: Plants may appear slightly yellow and have weakened growth. Roots appear to be dry, brown, and cracked and have a similar appearance to tree bark. Cracked areas usually occur in distinctive bands and may be swollen. Dark brown cracking may occur on the crown and taproot of the plant.

Black dot root rot: Roots are discolored, usually a honey-brown to grayish-brown, and are speckled with black dots.

Root knot: Roots are misshapen with small to large nematode-induced galls. Galls may range in size from pin-head to finger-sized. Golden-brown dots (egg masses) may appear on the outside of galls. Plants may appear stunted and weak.

Roots with severe corky root rot. Note the cracked, corky, bark-like roots.

Roots with sclerotia of Colletotrichum coccodes, the causal agent of black dot root rot. The honey brown discoloration is also characteristic of this disease.

 

Tomato roots with severe root knot nematode galling.

What causes these diseases?

Verticillium wilt is caused by the fungus Verticillium dahliae, which has an extremely broad host range. There are two races of V. dahliae that infect tomatoes.

Fusarium wilt is caused by the fungus Fusarium oxysporum f. sp. lycopersici and there are three races that infect tomatoes. The pathogen is an excellent soil survivor.

Corky root rot is caused by the fungus Pyrenochaeta lycopersici and survives in soil via microsclerotia that form on roots.

Black dot root rot is caused by the fungus Colletotrichum coccodes, which also causes anthracnose on tomato fruits. The pathogen is capable of surviving in soil by microsclerotia that form on infected fruit and roots (the black dots).

Root knot nematodes belong to the genus Meloidogyne. Both the northern root knot nematode (Meloidogyne hapla) and southern root knot nematode (Meloidogyne incognita) are present in Ohio tomatoes. In general, M. hapla forms smaller, distinct galls on tomato roots, while M. incognita tends to form larger, fused and malformed galls. Both species of nematodes have extremely broad host ranges.

How can I manage these diseases?

When managing soilborne diseases, growers should combine management practices that prevent existing soilborne pathogen populations from increasing with practices that actively reduce pathogen populations in the soil.

  1. Prevention: For transplant production, always use clean planting materials including seed, soilless media (recommended) and well or “city” water for irrigation. Ensure that seedlings are healthy before transplanting. Maintain proper fertility and watering to ensure healthy seedling development and maintain adequate nutrient and water levels throughout crop development.
  2. Sanitation: Remove diseased plants and diseased plant parts. Clean soil from boots and equipment between fields and high tunnels. Do not move from soilborne disease-affected fields to non-affected fields.
  3. Rotation: Rotate out of the same plant family when possible. For pathogens with extremely wide host ranges, such as Verticillium sp. and Meloidogyne spp., it is difficult to rotate to a suitable non-host crop. Since most soilborne pathogens are excellent soil survivors, rotations of 3-5 years are usually necessary to reduce pathogen populations adequately.
  4. Host resistance and grafting: Resistant varieties should be selected whenever possible and resistance to Verticillium wilt and Fusarium wilt is incorporated into most modern tomato varieties. Grafting a disease susceptible scion onto a disease resistant rootstock can reduce damage due to soilborne diseases. Many commonly used rootstocks have resistance to Verticillium wilt, Fusarium wilt, corky root rot, and some resistance to root knot nematode.
  5. Soil disinfestation: Several soil disinfestation options are available that vary in cost, efficacy, and environmental impact. Chemical fumigation and steam sterilization are two options that have been used historically, but are often not feasible for use on vegetable farms. Anaerobic soil disinfestation is a newer method of soil disinfestation that involves amending, saturating, and tarping soil. Soil solarization uses solar-generated heat trapped under plastic sheeting to kill soilborne pathogens, but this technique is not often effective under Midwestern conditions. Soils can be flooded or left fallow to kill pathogens over a period of time, but these methods are often ineffective due to the survival structures of most soilborne pathogens.
  6. Chemical or biological control: Few options are available and many biological control options are still experimental.

Article contributed by Anna Testen and Sally Miller, The Ohio State University Department of Plant Pathology

 

Downy Mildew Confirmed in Pumpkin and Zucchini in Ohio

Zucchini leaf with severe downy mildew. Small blocky lesions are yellow initially, then turn brown as leaf tissue dies. Diseased areas merge and eventually entire leaves die.

While we suspected that downy mildew might be present in southern or central Ohio based on weather conditions, storm trajectories and outbreaks in other states, we had not confirmed it until this weekend in pumpkins and today from zucchinis. The Pike County pumpkins were in a cultivar evaluation trial at the OSU-OARDC South Centers near Piketon. The zucchinis came from two organic farms in Guernsey County. The organic growers indicated that they had observed downy mildew symptoms about 2 weeks ago.

None of the fields had been treated with fungicides effective against downy mildew. As you can see in the zucchini leaf photos in this post, downy mildew can be very severe on squash; similar symptoms can occur on pumpkins. Fungicides should be applied before symptoms are observed – recommended fungicides can be found here.

Organic growers have limited options for downy mildew management. OMRI-approved copper-based fungicides are somewhat effective if applied preventatively. In the chart below, you can see that control varies by year –  in this case good in 2014 and poor in 2013. In both years, downy mildew was significantly less severe in cucumbers treated with Champ WG, oxidate or neem oil  than in non-treated control cucumber plants. Since these are all protectant fungicides, it is critical that they be applied before plants are infected with the downy mildew pathogen. In a separate study conducted at the University of Maryland (Everts & Newark, Plant Disease Management Reports 8:V210), Champ WG alternated with Serenade Soil or Actinovate was as effective as Champ WG alone in reducing downy mildew severity in both cucumber and muskmelon.

Click on chart to enlarge.

Are Pumpkin and Squash at Risk for Downy Mildew in Ohio?

Severe downy mildew on pumpkin, 2015.

Downy mildew has still not been confirmed on pumpkins and squash in Ohio, although the disease is widespread on cucumbers and melons throughout most of the state. This is the time of year when we really start to look for downy mildew on squash and pumpkins, as strains of the pathogen that attack these crops move into the area from the south, usually aided by remnants of hurricanes or tropical storms that reach Ohio. Remnants of tropical storm Harvey are likely to reach southern Ohio at least by Friday. Downy mildew spores can be transmitted long distances in the air, and are brought to the surface by rain.  We are particularly concerned with outbreaks reported in Kentucky on butternut squash this week (http://cdm.ipmpipe.org/scripts/map.php), an indication that the pathogen is established in the area and can serve as a source of inoculum for areas to the north and east.  Growers in southern Ohio should be especially vigilant, although we recommend stepped-up scouting of pumpkins and squash, and application of protectant fungicides, throughout the state.  Fungicide recommendations can be found in my June 28 post.

Cucurbit Downy Mildew Update – Two More Ohio Counties

Map of downy mildew reports in Ohio (all on cucumber or melon) and neighboring states, August 21, 2017. Red indicates reports within last 7 days. www.cdmipmpipe.org

Downy mildew was confirmed on cucumbers in two additional Ohio counties late last week: Harrison and Licking. We still have not confirmed downy mildew on pumpkins or squash in any part of Ohio.  On today’s Cucurbit Downy Mildew ipmPIPE conference call,  none of the representatives from nearby states (MI, WV, VA) reported seeing downy mildew on pumpkins or squash yet this season. However, downy mildew has been reported recently  on jack-o-lantern pumpkins in two counties in northwestern Indiana.  Careful scouting of pumpkin and squash fields should remain a priority.

Downy mildew can be confused with powdery mildew at very early stages of the latter (before the white powdery colonies appear), especially if the pale yellow areas on the top of the leaves are very dense.  Symptoms of other diseases including angular leaf spot and bacterial spot can also be confused with those of downy mildew.  If in doubt, please send a sample to your county educator or directly to the OSU Vegetable Pathology Lab in Wooster, OSU-OARDC, 1680 Madison Ave., Wooster, OH 44691; ph. 330-263-3838.

We are in the process of evaluating commercially available fungicides for efficacy against downy mildew using a bioassay. We will have results within a few weeks. However, we are not aware of new fungicide failures in other parts of the US this season, so our fungicide recommendations posted on June 28, 2017 remain in place.

 

More Ohio Counties with Cucumber and Melon Downy Mildew

Cucumber downy mildew

Downy mildew continues to take a toll on cucumbers and melons throughout the state. We have numerous reports of downy mildew on these two crops from northern and central Ohio, with new observations confirmed yesterday in Hardin and Greene counties. Downy mildew is best managed by preventative fungicide applications – a list of effective fungicides can be found in my June 28, 2017 post.

We still have not confirmed downy mildew on pumpkins or squash anywhere in Ohio.  However, downy mildew has been reported on butternut squash in Blair County in central PA, and on jack-o-lantern pumpkins in Starke County, in northwestern IN. It is a good idea to scout all cucurbits if you have not yet seen downy mildew in your area, and maintain a protectant fungicide program. Keep in mind that on squash and pumpkins, powdery mildew can look like downy mildew in the early stages before the white powdery mycelium is seen. Many of the fungicides used to manage these two very different diseases are fairly specific to either one, so be sure to have an accurate diagnosis.

Mild Temps, Clouds and Rain = Increased Late Blight, Downy Mildew Risk

Late blight on tomatoes. Photo by Wayne County IPM Scout Chris Smedley.

Late blight has been confirmed on tomatoes in Wayne and Ashland counties this week. We also found downy mildew in basil, cucumber and melons in our sentinel plots in Fremont and Celeryville, OH. Downy mildew is assumed to be widespread on cucumbers and melons in Ohio, but we have not yet found downy mildew on squash or pumpkins. The late blight and downy mildew pathogens are related and all are favored by moderate temperatures, rain, high humidity and cloudy skies. Tomatoes, potatoes, cucurbits and basil should be scouted regularly and treated with protectant fungicides. Management recommendations have been posted this summer for all of these diseases.  See the following:

Late blight: conventional and organic management

Cucurbit downy mildew management

Basil downy mildew management

 

New Reports of Cucumber Downy Mildew in Central Ohio

Downy mildew symptoms on upper surface of a cucumber leaf.

Cucumber downy mildew was confirmed in Delaware and Franklin counties this week.  Cucumber and melon growers in northern and central Ohio should assume that downy mildew risk is high and should protect their plants with fungicides.  Since downy mildew has also been reported on cantaloupe and cucumber in Kentucky (Lexington area), it is likely that theses crops are also at risk in southern Ohio.  A list of recommended fungicides was posted here on June 28. Additional information can be found in the Midwest Vegetable Production Guide for Commercial Growers.

We still have not confirmed downy mildew on squash or pumpkins in Ohio, but we continue to monitor our sentinel plots, field research plots and commercial fields. We will be happy to check out a pumpkin or squash sample, or any cucurbit sample, for downy mildew – please send affected leaves in a box by priority mail or 1- or 2-day courier, or drop them off at out lab: Vegetable Pathology Lab, OSU-OARDC, 1680 Madison Ave., Wooster, OH 44691; ph. 330-263-3838.

Cucurbit Powdery Mildew: A Little Late this Year but Start Scouting Now

We are just now finding powdery mildew on squash and pumpkins, several weeks later in the season than we have seen it during the past few years. So far disease incidence and severity have been relatively low in commercial fields, as well as in OSU research plots.  The fungus that causes cucurbit powdery mildew does not overwinter in Ohio, so the disease does not appear until spores arrive on wind currents from warmer growing areas.  This fungus is an unusual plant pathogen in that it is inhibited by free water – so the frequent rains we have been experiencing may have kept this disease at bay for the time being. However, it is here now and will undoubtedly flare up in susceptible cucurbits unless they are treated with fungicides. Signs of infection are small circular powdery growths (mycelium and spores of the pathogen) on either side of the leaf. These spots enlarge and can eventually cover most of the leaf surface and kill the leaves.  Stems and leaf petioles are also susceptible, but the disease is not observed on fruit.  In pumpkins, powdery mildew may also attack the “handles”, which can be further damaged by secondary pathogens.

Click on table to enlarge.

Powdery mildew is managed using powdery mildew-resistant varieties and fungicides.  Development of insensitivity to overused fungicides is common in populations of the fungus that causes this disease, so it is important that a fungicide resistance management program is followed. Remember to alternate fungicides in different FRAC (Fungicide Resistance Action Committee) groups, indicating different modes of action against the fungus. It is important to apply fungicides when the disease first appears and incidence is low. Fungicides that are effective against cucurbit powdery mildew can be found in the Midwest Vegetable Production Guide for Commercial Growers; product ratings are on page 117.  Our evaluations of efficacy of powdery mildew fungicides at three locations (Wooster, Columbus, South Charleston) in Ohio in 2016 indicated that five products consistently provided very good control (> 93%) of powdery mildew  on pumpkins in all three locations (see table).  Three products were very good in Wooster and Columbus but fair in South Charleston; control by Fontelis was 73% in South Charleston, while the others provided less than 55% control at that location.  Both Bravo and Pristine performed poorly in all three locations.

Cucumber Downy Mildew Confirmed in Southeastern Ohio – Belmont County

We confirmed downy mildew in cucumbers collected yesterday from a small commercial field in Belmont County, near Barnesville.  Disease incidence and severity were high in this field, which had not been treated with any fungicides. The outbreak followed several days of intense rainfall in the previous week.

Map of cucurbit downy mildew outbreaks – CDM ipmPIPE, July 26, 2017.

This is the first report of cucumber downy mildew in central or southern Ohio, and was found during a field walk sponsored by OSU Extension and the Captina Produce Auction.  Ohio growers should assume that cucumber/melon downy mildew is more widespread than we have been able to report, and should protect these crops with appropriate fungicides as listed in my post on July 28.  We have not seen downy mildew on squash, pumpkins or watermelon in Ohio to date, and the downy mildew pathogen population currently affecting cucumbers and melons in Ohio is not likely attack pumpkins or squash.  A pumpkin field near the cucumber planting that was highly diseased was not affected, and we have not found downy mildew in our sentinel plots, despite the presence of infected cucumbers for several weeks.  However, downy mildew populations that can damage pumpkins and squash are likely to move in from the southeastern U.S. later in the season.

We depend on county educators, growers and consultants to let us know when cucurbit downy mildew is suspected, particularly in counties where it has not been reported.  Our lab will diagnose samples at no cost to Ohio growers, so we appreciate receiving samples that, if downy mildew is confirmed, will enable us to alert the cucurbit growing community.

 

 

 

Downy Mildew Confirmed on Melons in Wayne County and Cucumbers in Henry County, OH

Downy mildew on cantaloupe.

Downy mildew continues to spread on cucumbers in Ohio, with a confirmed report in Henry County this week.  The disease is likely to be widespread on cucumbers in northern Ohio, particularly after last week’s rainy, humid weather.  As usually happens within a few weeks of cucumber downy mildew outbreaks, we are now finding downy mildew on cantaloupe.  Chris Smedley and the Wayne County IPM Scouting team found widespread and fairly severe downy mildew in commercial melons in the northwestern part of Wayne County, where we first reported downy mildew on cucumbers on June 28.  As noted in previous posts, it is imperative that growers protect melons as well as cucumbers with downy mildew-effective fungicides such as Orondis Opti and Ranman if they are in an area where downy mildew risk is high, such as northern Ohio.  See my June 28 post for a list of recommended fungicides. Remember to follow label instructions and alternate products with different modes of action.