Andy Wyenandt

This is an archive of Dr. Wyenandt's posts on the Plant and Pest Advisory.

July 8, 2022 Andy Wyenandt

Phytophthora and Pythium control during wet weather

Most of New Jersey has been plagued by heavy regular rains and pop-up thunderstorms making conditions ideal for pathogens such as Phytophthora and Pythium. Unfortunately, Pythium and Phytophthora blight can be found on most farms in the southern part of the state. Poor crop rotations with susceptible hosts only make matters worse. The Phytophthora pathogen has an increasing host range that now includes snap and lima beans; and all crops, other than a few resistant bell pepper cultivars, lack any resistance to the pathogen.

Control of Phytophthora blight and Pythium are extremely difficult (even with the use of fungicides) in the wet weather conditions. In the past few years a number of new fungicides, with new active ingredients, have become commercially-available for use on multiple crops. Mefenoxam or metalaxyl, both once widely-used to effectively control Phytophthora blight has been hit by resistance issues around much of Southern New Jersey the past decade. Growers with a known history of mefenoxam-insensitivity on their farm should use Presidio, Previcur Flex, or Ranman plus a Phosphite fungicide in rotation in their drip application programs. Importantly, if mefenoxam has not been used in particular fields on any crop for a number of years (more than 5+) the fungus may revert back to being mefenoxam-sensitive and control with these products may return. Mefenoxam, metalaxyl, Previcur Flex, and the phosphites are the most systemic of the group and should readily be taken up the by plant via application through the drip. Presidio has locally systemic and has translaminar activity and should offer some protection of the root system via drip. Ranman has protectant activity and thus will offer some root protection where it comes into contact with. Orondis Gold (oxathiapiprolin + mefenoxam, 49 +4) is the newest fungicide available with a new active ingredient in a new FRAC group. Additionally, in past research trials, mefenoxam, Orondis Gold, Presidio, Previcur Flex, Ranman, Revus and the phosphites in rotation and/or tank mixes have offered very good control of the fruit rot phase of phytophthora blight.
[Read more…]

Filed Under: Vegetable Crops Tagged With: Phytophthora blight

July 7, 2022 Andy Wyenandt

Controlling fungal leaf blights of Carrot

Powdery mildew, Alternaria and Cercospora are three important fungal foliar pathogens that can cause early defoliation in carrots, thus reducing yields and making harvest difficult. Each pathogen produces distinct symptoms. [Read more…]

Filed Under: Vegetable Crops Tagged With: Carrot, leaf blights, powdery mildew

July 6, 2022 Andy Wyenandt

Diagnosing Southern blight and White mold in tomato and pepper

There have been a few reports of Southern blight (Sclerotinia rolfsii) and White mold (Sclerotinia sclerotiorum) on tomato and pepper in New Jersey. Southern blight is much more common in vegetable areas south of the state where summer temperatures remain hotter (above 90°F) for longer periods of time. Like white mold, it can survive in the soil for many years. Symptoms of Southern blight include infection at the base of the stem at the soil line. The resulting infection will girdle the plant causing wilt and death. The fungus will produce white, cottony mycelium and very small, spherical sclerotia which are often have a tannish, brown color.

White mold is more common than Southern blight in New Jersey, and like Southern blight, once introduced into a field or high tunnel it can very difficult to control. The pathogen produces large black sclerotia on the surface and inside infected stems. If sclerotia of either pathogen make their way back into the soil, both can survive for years causing significant problems.

All infected plants need to be removed immediately and disposed of properly to help reduce the chances of sclerotia returning to the soil.

For more information on chemical control please see the 2022/2023 mid-Atlantic Commercial Vegetable Production Recommendations Guide.

Symptoms of Southern blight on infected pepper plant. Note the numerous, small white to tan colored sclerotia on the stem.

White mold of tomato. Note the large black sclerotia developing inside the brittle stems.

Filed Under: Vegetable Crops Tagged With: pepper, southern blight, tomato, white mold

July 5, 2022 Andy Wyenandt

Avoiding Plectosporium blight in cucurbit fields in 2022

Plectosporium blight, also known as Microdochium blight or White speck, caused significant problems in some pumpkin fields last summer in New Jersey. The soil-borne fungal pathogen, although somewhat uncommon, can unexpectedly show up in some years and cause significant losses if left uncontrolled. The fungus survives in the soil on decaying plant debris where it can remain saphrophytic by surviving off organic matter. Infection is characterized by the production of numerous light tan to “bleached” spindle shaped lesions that develop on vines and the undersides of infected leaves. Heavily infected vines and leaves can die leading to premature defoliation and subsequent sunscald on fruit. In cases of heavy disease pressure, spores that are produced on the bottom sides of leaves fall and infect the topsides of fruit laying beneath the canopy. Infection of stems leads to premature browning and drying reduce their longevity. Fruit infection, in most cases, remain mostly cosmetic in nature reducing fruit quality and may predispose fruit to other opportunistic fruit rots. Plectosporium blight often shows up during periods of prolonged wet weather where the soil remains wet for extended periods. “Hot spots” typically appear in fields before the pathogen is further spread by driving rains and wind.

From a production standpoint, stay away from fields with known history of the disease for as long as possible; provide adequate spacing between plants in- and between rows (i.e., avoid the overcrowding of plants); avoid over (preplant) fertilization that can lead to thick, dense canopies; avoid overhead irrigation (if possible); avoid planting in area of a field that remains heavily shaded where soils tend to dry too slow.

Controlling Plectosporium blight begins with regular scouting, recognizing symptoms, and identifying “hot spots” in the field. Protectant fungicides, such as chlorothalonil, as well as those used in weekly maintenance spray programs for cucurbit powdery mildew control will help control Plectosporium blight as long they applied on a weekly schedule with a high volume of water with thorough coverage. To help improve control on the undersides of leaves, a FRAC code 11 fungicide such as Quadris Top or Pristine, can be added to the tank mix. Remember, FRAC code 11 fungicides have translaminar activity and will move from the top surface of the leaf to the bottom. Growers who grow powdery mildew resistant varieties need to remember to scout their fields regularly even if cucurbit powdery mildew has not been detected on the farm or if regular maintenance sprays haven’t begun.

White speck lesions covering the surface of immature and mature pumpkin fruit. White speck will only cause cosmetic injury to fruit.

White speck on vine and stem of infected pumpkin plant. Note the numerous small, white diamond shaped lesions.

Filed Under: Vegetable Crops Tagged With: pumpkin, White speck

July 4, 2022 Andy Wyenandt

Destroying cucurbit plantings after harvesting is a must to help mitigate downy mildew

Now is a good time to start thinking about ways to help mitigate cucurbit downy mildew pressure later in the production season. All cucurbit fields, especially cucumber and other summer squash fields should be destroyed immediately after the last harvest to help limit potential inoculum sources for cucurbit downy mildew. For a review of CDM control please click here. To track the progress of CDM on your phone or PC please visit the CDM forecasting website.

Filed Under: Vegetable Crops Tagged With: cucurbit downy mildew

July 3, 2022 Andy Wyenandt

Diagnosing important diseases in Cucurbit crops – Reference Guide

The following images consist of important diseases in cucurbit crops. These images can be used as a quick reference for diagnosing important fungal and bacterial pathogens. For best results, please turn your device (i.e., cell phone) sideways. For information on commercial control recommendations, please visit the cucurbit sections of the 2022/2023 Mid-Atlantic Commercial Vegetable Production Guide.

Cucurbit powdery mildew (CPM)	Cucurbit powdery mildew	CPM on infected pumpkin stem

Cucurbit downy mildew (CDM)	CDM sporulating on underside of infected cucumber leaf	CDM sporulating on underside of infected cucumber leaf with 10x hand

Anthracnose on cucumber	Anthracnose lesion on infected leaf vein	Anthracnose-infected cucumber fruit

Plectosporium blight on infected stem	Plectosporium lesions on infected vines	Plectosporium lesions on infected pumpkin fruit

Phytophthora blight symptoms	Phytophthora-infected squash	Phytophthora-infected watermelon field

Phytophthora-infected pumpkin	Phytophthora-infected squash fruit	Phytophthora-infected watermelon fruit

Mosaic virus symptoms on leaves of infected pumpkin plant	Pumpkin fruit infected with mosaic virus	Virus infected squash fruit

Fusarium fruit rot of pumpkin	White mold on infected fruit	Gummy stem blight

Choanephora-infected fruit	Choanephora spores in infected pumpkin fruit	Choanephora infecting giant pumpkin fruit

Bacterial wilt infected plant	Symptoms of bacterial canker infected stem	Damage done by cucumber beetle feeding

Angular leaf spot	Shot holes in pumpkin leaves caused by Angular leaf spot in pumpkin	Mature pumpkin fruit with sunscald injury due to the loss of plant canopy