Managing Phytophthora Blight in Peppers

Phytophthora blight has become a very serious problem in peppers and cucurbits, particularly in areas with concentrated vegetable production.  The pathogen is a water mold that thrives under conditions of high moisture and high temperature. It produces motile spores (zoospores) that are attracted to plants, then form a structure that allows them to infect, and aggressively attack any type of plant tissue. Zoospores can be splashed onto leaves, stems and fruits during rain events and overhead irrigation. Phytophthora blight is often seen first in low spots or other poorly drained areas of production fields, but the disease also occurs on well-drained, even sandy soils if the environmental conditions are right.  An increase in intensive rainfall events that result in soil saturation and standing water in fields in the last decade or so is certainly a contributing factor to the uptick in problems with Phytophthora blight.

Effective management of Phytophthora blight in peppers requires an integrated approach:

Crop rotation.  Phytophthora produces structures called oospores that can survive for a number of years in the soil.  Plan to rotate out of peppers, cucurbits or green beans for 4-5 years if Phytophthora blight has been a problem.

Resistant varieties.  A few pepper varieties are resistant to the root rot phase of the disease.  In general, these varieties are susceptible to the crown rot phase, which affects foliage and fruits. Varieties with moderate to good resistance to Phytophthora blight are: Paladin, Aristotle, Declaration, Intruder, Vanguard (bell); Hechicero (jalapeño); and Sequioa (ancho).

Well-drained soil. Avoiding standing water is critical to limiting the movement of Phytophthora from plant to plant.

Avoid surface water for irrigation. We have found Phytophthora in irrigation ditches and ponds as early as late June in vegetable production-intensive areas in Ohio.  Using surface water for irrigation is risky, especially if Phytophthora is present in fields near surface water sources.

Plant on raised beds. Prepared properly, raised beds will help prevent standing water near pepper plants.  If possible beds should be domed, and there should be no depressions in the soil surrounding the plants.

Sanitation.  Phytophthora can be moved from an infested field to a clean one on soil clinging to boots, equipment, etc.  Power washing to remove soil is a good first step, followed by rinsing with a sanitizer.

Fungicides.  There are a number of fungicides labeled for use on peppers to manage Phytophthora blight  (see table below).  The newest product, Orondis, has very good efficacy against this disease.  It is available in the Midwest this year as a co-pack with either Revus (Orondis Ultra), Ridomil (Orondis Gold) or Bravo (Orondis Opti).  Pre-mixes will be available in 2018. There are many restrictions on the use of Orondis – including the number of applications (no more than 1/3 of total applications for Phytophthora blight) and when it can be applied (to the soil or to the foliage but not both).  Orondis Ultra and Orondis Gold can be applied in transplant water or through the drip, although Orondis does not move much in soil and emitters need to be right next to the plant.  If the pepper variety is susceptible to Phytophthora blight, it may be a good idea to apply Orondis Gold or Orondis Ultra at planting, and follow up later with a program containing at least two of the fungicides with activity against Phytophthora (see table).  Research conducted at the University of Illinois has shown that adding a copper-based fungicide to these foliar applications can improve their efficacy.  If the pepper variety is resistant to Phytophthora, any of the three Orondis products can be used in a foliar fungicide program that includes other effective fungicides.  The Bravo component of Orondis Opti will not help with Phytophthora blight, but will control anthracnose.  Orondis Gold is considerably more expensive than Orondis Ultra and Orondis Opti, and resistance in Phytophthora to the Ridomil component of Orondis Gold has been found in numerous locations.

What Are the Most Worrisome Diseases in Your Vegetable Crops?

Many specialty crop growers are aware of the USDA IR-4 Minor Use Program, which works to promote registration of products for  pest, disease and weed management in “minor use” crops – including vegetables, fruits, herbs and spices, among others.  The US IR-4 Program will participate in a Global Minor Use Summit in Canada later this year, and is asking for help in identifying the most important priorities among growers worldwide.  This is a good opportunity to voice your opinion about the diseases, pests and weeds that are your greatest concern and for which labeled products are needed to manage the disease.  In some cases this is a matter of expanding labels that include other crops, while in others it may involve research to find a solution to a problem.

For example, bacterial diseases such as bacterial spot and bacterial canker in tomatoes, are very difficult to manage in the field, and therefore can cause significant losses.  There are no highly effective products available, so research is needed.  

Please take the time in the next ten days to let us know what you think are the most important crop/disease, pest or weed priorities on your farm or community.  Which are the ones that keep you awake at night? You can send me an email at miller.769@osu.edu or reply in the Comment section below.

Basil Growers – Be on the Watch for Downy Mildew

Basil plants in a nursery in Ohio were confirmed to have downy mildew by Nancy Taylor of OSU’s Plant and Pest Diagnostic Clinic (Fig. 1).  The pathogen that causes basil downy mildew, Peronospora belbahrii, is seed borne and favored by cool, humid/rainy conditions.  Overhead irrigation in the greenhouse/nursery promotes the development and spread of the pathogen, as does rainfall outdoors.  Symptoms on leaves start with diffuse yellowing and browning that can look like sunburn.  Leaf lesions later become black (Fig. 2).  A diagnosis of downy mildew is made when spores of the pathogen are observed on the underside of lesions (Fig. 3); the best time to look for these dark purple-black structures clustered together is early in the morning.

Fig. 1. Downy mildew symptoms on basil seedlings

Fig. 2. Black lesions on upper surface of leaf of basil plant.

 

 

 

 

 

 

 

Fig. 3. Spores (sporangia) of the basil downy mildew pathogen on the lower surface of a basil leaf lesion.

There are differences in susceptibility of basil varieties and types to downy mildew, with sweet basil generally the most susceptible.  Fungicides are available for downy mildew management in basil – see Dr. Meg McGrath’s article for recommendations for both conventional and organic systems. However, as is the case for the closely related cucurbit downy mildew, fungicides work best when applied preventatively – before symptoms are observed.  Many, but not all of the fungicides are allowed for use in greenhouses or other protected culture systems such as high tunnels.  See page 45 of the 2017 Midwest Vegetable Production Guide for Commercial Growers for a listing of allowed uses of fungicides for protected culture.

If you suspect downy mildew in your basil crop, you may send or drop off samples to the OSU Plant and Pest Diagnostic Clinic in Reynoldsburg or the OSU Vegetable Pathology Lab in Wooster (call, text or email Sally Miller (miller.769@osu.edu)/330-466-5249 or Francesca Rotondo (rotondo.11@osu.edu/865-640-5621).

 

OSU Vegetable Disease Management Research Report for 2016 Available

Our 2016 final report on field research trials conducted in 2016 can be accessed here:plantpathologyseriesno-146.  The hot dry weather for most of the summer of 2016, followed by a wet, fairly warm late summer/early autumn, created some challenges, but we have different sorts of challenges every year!  One advantage of low to moderate amounts of rainfall, compared to the deluges we experienced in northern Ohio in previous summers, is that bacterial leaf spot pressure in tomatoes was not overwhelming, and we could see some product effects.  The high temperatures were very favorable (too favorable) for Phytophthora blight in squash, and really put those products to the test.  The weather didn’t matter for our trial for powdery mildew control in greenhouse cucumbers – some nice results are reported.  Here are some highlights:

Fungicides to control powdery mildew in greenhouse cucumbers: Weekly treatments of Terraguard SC (8 fl oz) (left photo) and Pyriofenone 300SC (4 fl oz) (center photo) controlled powdery mildew and increased cucumber fruit yield compared to the non-treated control (right photo). There was some crop injury with the Terraguard treatment. Interestingly, powdery mildew severity remained low 2 weeks after the last treatment for both fungicides. 
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Bioassay for sensitivity of the powdery mildew pathogen to fungicides: OSU graduate student Claudio Vrisman conducted a bioassay to evaluate the sensitivity of the powdery mildew pathogen, Podosphaera xanthii, to different fungicides in three locations in Ohio. Young squash squash-pm-control-chartplants were produced in a greenhouse in Wooster, treated with one of the test fungicides, and placed next to pumpkin fields with plenty of powdery mildew in Wooster, South Charleston, or Columbus.

Several fungicides performed better in Wooster and Columbus than in South
Charleston (see chart – click to enlarge).  Bravo Weather Stik was not effective in any location and Pristine provided only partial control in all three locations. Aprovia Top, Inspire Super, Procure, Quintec, and Rally provided >92% control in all locations. These results indicate that the efficacy of some fungicides may differ in different locations, and it is critical to manage fungicide use according to label instructions to minimize the development of fungicide insensitivity in powdery mildew pathogen populations.

Evaluation of Orondis and other fungicides to control Phytophthora blight in squash: Squash and pumpkins are very susceptible to Phytophthora blight, which has become a widespread problem in Ohio in the last two decades.  There are no commercial squash or pumpkin varieties available with a satisfactory degree of resistance to this disease, and most fungicides are also not highly effective in squash and pumpkins. The new Syngenta Crop Protection fungicide Orondis is very effective against related diseases such as late blight and downy mildew, and has shown promise against Phytophthora blight in peppers.  We tested eleven different combinations of Orondis and other fungicides active against this group of pathogens. In 2016, Orondis came in three co-packs, paired with Revus (Orondis Ultra), Ridomil Gold (Orondis Gold), or Bravo (Orondis Opti) – more on new formulations for 2017 in a later post.  The number of applications is limited to 33% of the total fungicide applications for Phytophthora blight, and Orondis soil applications cannot be followed by Orondis foliar applications.

orondis-phytoph-chart-2016We conducted the trial in Fremont at the OSU-OARDC North Central Agricultural Experiment Station.  Phytophthora blight was severe in this trial – 98% of the plants in the non-treated control were killed by August 4.  As you can see in this chart (click to enlarge), none of the fungicide combinations were highly effective against Phytophthora blight. Only four treatments (green bars) significantly reduced the number of wilted/dead plants compared to the non-treated control. Although numerically a little different, there are no significant differences among these four treatments.  The Orondis Gold drench does not appear to add any additional value in the control of Phytophthora blight over Orondis Ultra foliar applications alternated with Ranman or Presidio. We conducted a second trial at the OSU-OARDC Muck Crops Experiment Station in Celeryville, OH, but the disease pressure was so severe that treatment differences could not be detected.  Effectively managing Phytophthora blight in squash and pumpkins will require an integrated approach that combines cultural practices such as water management and site selection (fields with low populations of the pathogen) and fungicides.

• Evaluation of products to manage bacterial spot in processing tomatoes: We conducted two bacterial spot trials in 2016 (the second one can be found at plantpathologyseriesno-146).  This trial was designed to test the efficacy of several commercial products in management of bacterial spot.  Since 2016 was dry for most of the summer, bacterial spot developed slowly and overall, disease pressure was moderate.  Under these conditions we observed significant differences in foliar bacterial spot severity and in the percentage of fruit with bacterial spot symptoms, compared to the non-treated control (see charts below).

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Alternaria Leaf Spot of Cauliflower

We have received several samples of cauliflower in the last two weeks with black spots on the heads. These spots ranged from small pinpoint dots to larger circular spots up to several mm in diameter (Fig. 1). Many of the blighted curds also had light brown, soft, slimy spots, which often coalesced into larger zones

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Figure 1. Cauliflower head with prominent black lesions caused by Alternaria brassicicola.

that penetrated well into the heads (Figs 2 and 3). The black spots were caused by either Alternaria brassicae or A. brassicicola. The lighter brown slimy spots were caused by the soft rot pathogen, Pectobacterium carotovorum subsp. carotovorum. Since Pectobacterium infects wounded or damaged plant tissues, it is likely that the soft rot occurred secondarily to the Alternaria infections, although soft rot may be found in the absence of Alternaria. Alternaria also causes round to ovoid bull’s eye lesions on leaves and stems of cauliflower. Yellowing of tissues surrounding the lesions, as well as leaf drop, may also occur. While Alternaria lesions generally do not progress deeply into the cauliflower head, they are unsightly and render the heads unmarketable. Secondary soft rot infections can cause significantly more damage to the heads than the initial Alternaria infection.

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Figure 2. Soft rot (light brown) and Alternaria (black) spots on cauliflower head.

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Figure 3. Soft rot caused by Pectobacterium on orange cauliflower.

Alternaria brassicae and A. brassicicola can cause disease in any brassica (also called crucifers or cole crops), but not plants outside this family. They survive between seasons in crop debris and on seeds. They produce large amounts of spores that travel on the wind from plant to plant. Warm temperatures and high relative humidity favor disease development. It is important to adopt an integrated approach to managing this disease, focusing on clean seed, cultural practices and effective fungicides used in accordance with label requirements for resistance management.

  1. Buy certified disease-free seeds or treat seeds with hot water. Always check with your seed supplier to determine if seeds have already been hot water-treated. Some brassica seeds are routinely treated with hot water for black rot control. Seeds should never be hot water-treated more than once.
  2. Scout seedlings in the greenhouse or seedbed and remove those with symptoms.
  3. Rotate out of brassicas for several years to allow enough time for Alternaria inoculum loads in the soil to drop to low levels.
  4. Incorporate crop debris into soil to hasten decomposition.
  5. Do not maintain cull piles of brassicas – compost or bury culls to destroy Alternaria spores.
  6. Control weeds in the Brassica/Crucifer family.
  7. Manage irrigation to allow plants to dry before evening.
  8. Apply fungicides as suggested in the Midwest Vegetable Production Guide for Commercial Growers. Be sure to alternate fungicides with different modes of action to reduce the risk of fungicide resistance development.

Should cauliflower be sprayed now?  It depends – if the weather will continue to be warm (60-78 F) in your area and you anticipate having the cauliflower crop in the field for a few weeks, it may be worth the cost of fungicides if the the disease is in the early stages.  Most fungicides recommend beginning applications preventatively or when the disease first appears.  Note that Preharvest Intervals (PHIs) range from 0 to 7 days for fungicides labeled for use on cauliflower.

 

 

Downy Mildew Now Widespread in Ohio Cucurbits

Our first report in 2016 of downy mildew on pumpkins in Ohio came from Pike County, in the southern part of the state. Late last week we found downy mildew on pumpkins and squash in our sentinel plot in Wooster (Wayne County) for the first time this year. At the same time we found downy mildew in pumpkins in research plots at Waterman Farm on the OSU campus (Franklin County) and OSU-OARDC’s Western Agricultural Research Station in South Charleston (WARS; Clark County).  We also found downy mildew on cucumbers at the WARS and Waterman Farm locations for the first time this year.

Our monitoring of downy mildew outbreaks is not complete – while we frequently check our sentinel and research plots in several locations and diagnose samples from multiple Ohio locations, we haven’t confirmed the disease in most Ohio counties despite the fact that it is likely to be present.  Nonetheless, these observations confirm what we have been seeing the last few years – there are likely at least two populations of the downy mildew pathogen in Ohio.  These populations probably have different sources and different host ranges.

The northern population has a high affinity for cucumbers and melons and appears in northern Ohio by the 4th of July in most years (blue oval on map below). This population does not appear to cause disease on pumpkins, squash or watermelon.  In our sentinel plots in northern Ohio, we never see downy mildew early in the summer on the pumpkins, squash or watermelon, even

when the cucumbers in the same field are nearly dead and melons quite damaged due to downy mildew.   This population is likely surviving over the winter on cucumbers in greenhouses in the Great Lakes region.  The southern population has a high affinity for squash and pumpkins – it appears later in the season – often not until mid-to-late August or early September (pink oval on map). It is probably arriving from the southeastern US on late summer/early fall storms related to hurricane activity, although sources to the west are also possible.  This  population usually appears first in southern Ohio and works its way up to central and eventually northern Ohio.  Where the two ovals overlap in central Ohio, we tend to find downy mildew on all cucurbit types at about the same time.  We don’t know if this is due to movement of the northern type into central Ohio, or a broader host range of the southern populations.

This means that pumpkin and squash fields needing to maintain foliage for additional time this season should be scouted and fungicides effective against downy mildew should be applied if the disease is found.  It is also a good idea to apply fungicides preventatively if downy mildew has been confirmed nearby.  See the September 2 post for fungicide recommendations.

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A very rough estimate of downy mildew populations in Ohio. Note overlap of the northern and southern populations in central OH.

 

Downy Mildew Now on Pumpkins in Southern Ohio

We have confirmed downy mildew on pumpkins for the first time in Ohio this year, in Pike County. Downy mildew is widespread in northern Ohio on cucumbers and melons, but has not been reported on pumpkins, squash or watermelon in this area to date. The Pike County report of downy mildew on pumpkins comes less than 2 weeks after it was reported on processing pumpkins in Illinois. The only way to control this disease is through the use of effective fungicides, applied preventatively. Once the disease becomes obvious in a field, it may be too late to get good control. Whether or not downy mildew should be controlled on pumpkins as we enter into September and later is up to the grower. The disease does not cause symptoms on fruit, so if pumpkins are at or near the stage to be harvested, it may not be necessary to apply fungicides. If fungicides are to be used, we recommend Orondis Opti alternated with Ranman,  Zing! or Zampro (tank mixed with a protectant fungicide like chlorothalanil or mancozeb if not included in the product). We are also finding bacterial spot on pumpkins, so growers should confirm downy mildew before applying these fungicides.

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Downy mildew symptoms on a pumpkin leaf.

Tospoviruses in Tomatoes and Peppers

Thanks to Dr. Celeste Welty, OSU Department of Entomology, for contributing to this post.

We have seen an unusually high number of tomato and pepper samples with Tomato spotted wilt virus and related Tospovirus diseases this spring and summer. Symptoms on foliage include necrotic and/or chlorotic spots, and necrotic spots or streaks on stems and petioles. Ringspots may be observed on leaves. Plants tend to be stunted and may wilt. Fruits develop a range of symptoms, from chlorotic blotches to ringspots.

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TSWV on pepper – notice necrotic spots on leaves.

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Closeup of TSWV on pepper leaf – note ringspot symptoms.

Tospoviruses are transmitted from plant to plant by thrips. Thrips larvae feeding on plants acquire the virus and become infective for the lifetime of the insect. The viruses overwinter in infected plant debris and weeds. Plants infected at the seedling stage are likely to have more severe symptoms than plants infected later.

These viruses are managed differently than aphid-transmitted viruses because they have a longer development cycle within the thrips, so there is hope of getting the problem under control if prompt action is taken. If isolated plants show symptoms, these should be rogued and destroyed as soon as possible. Insecticides can be effective if applied as soon as the problem is diagnosed, however, note that thrips are one of the more difficult pests to kill with our current choices of insecticides. We have Radiant (spinetoram), Movento (spirotetramat), and Exirel (cyantraniliprole) that generally do the best; beware these are quite expensive. Lannate (methomyl) can do ok; the pyrethroids (such as Warrior, Mustang, Baythroid, Asana) are generally poor for thrips control. Movento is systemic but needs an adjuvant such as Dyne-Amic or LI-700 to get it into the plant. Note Radiant and Movento are not allowed in greenhouses or high tunnels.

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TSWV symptoms on green tomato fruit (cluster type) in greenhouse (photo courtesy of Anna Testen).

Cucurbit Downy Mildew Update

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Counties in which downy mildew has been reported on cucurbits. Most recent outbreaks are in red. Map from cdmipmpipe.org.

So far downy mildew has been confirmed on cucumbers in Wayne, Huron, Sandusky, Medina, and Fulton counties in Ohio. We also found it on cantaloupe in our sentinel plots in Huron County on July 28. We have not had reports of downy mildew on squash or pumpkins in Ohio to date. With dry or very dry weather in much of Ohio (1/3 of Ohio is currently under moderate drought conditions), we expect that downy mildew will continue to be less frequent and less severe than in previous years. However, in gardens and fields where it has occurred and fungicides have not been applied, downy mildew has been very damaging.  We are recommending Orondis Opti alternated with Ranman, Gavel, Zing! or Zampro (tank mixed with a protectant fungicide like chlorothalanil or mancozeb if not included in the product). Presidio has been an effective fungicide in the past, but several studies have shown that it has lost efficacy in the last few years. Growers should be aware, especially with pumpkins, of other diseases that may be confused with downy mildew. Bacterial spot of pumpkin can cause leaf spots that converge and kill the leaves; however, fungicides are ineffective against bacterial spot. Despite the dry weather, we have seen bacterial spot of pumpkin and other bacterial diseases of vegetable crops this year in Ohio. Bacterial diseases are best managed in vegetables by using clean seed and reducing bacterial populations during transplant production by limiting moisture and applying bactericides.

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Bacterial spot symptoms on pumpkin leaf.

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Downy mildew symptoms on pumpkin leaf.

 

 

Blackleg Caused by Dickeya dianthicola confirmed in OH

A potato sample with blackleg symptoms received late last week in the OSU Vegetable Pathology lab was confirmed to be caused by Dickeya dianthicola.  This is the first report of the more aggressive type of blackleg in Ohio.  In hot weather, the disease can cause significant losses in potatoes. Growers who find blackleg symptoms in potatoes are welcome to submit a sample to the OSU Vegetable Pathology Lab (scroll below to June 29 post for sample submission instructions).

There are no strategies to manage blackleg once it appears in production fields.  The disease must be controlled at the seed production stage.  For more information, see Dr. Meg McGrath’s article from Cornell University