NC State Extension Publications

General Information

Production of tomatoes in protected environments, such as greenhouses and high tunnels, has gained in popularity in the last decade and requires a different approach to disease management than field production. Diseases such as bacterial spot and speck, Septoria leaf spot, and early blight are easier to manage in greenhouses and high tunnels because plants are protected from heavy rainfall events, which promotes these diseases. However, these structures are prone to high humidity and tend to be wind free, which can encourage disease problems that are uncommon in field production such as leaf mold and powdery mildew. Cool temperatures are conducive for Botrytis gray mold development and are common during spring greenhouse and high tunnel production.

Gray mold caused by Botrytis cinerea on high tunnel tomatoes

Gray mold caused by Botrytis cinerea on high tunnel tomatoes

NCSU Plant Disease and Insect Clinic

Leaf mold caused by Passalora fulva on tomato.

Leaf mold caused by Passalora fulva on tomato.

R. Melanson, Mississippi State University

Powdery mildew caused by Oidium spp. on tomato.

Powdery mildew caused by Oidium spp. on tomato.

NCSU Plant Disease and Insect Clinic

Pathogens

Gray mold is caused by the fungal pathogen Botrytis cinerea. Notable vegetable high tunnel host crops include tomato, pepper, lettuce, and cucumber.

Leaf mold is caused by the fungal pathogen Passalora fulva (syn. Fulvia fulva). Notable vegetable high tunnel host crops include tomato, pepper, and eggplant.

Powdery mildew on tomato can be caused by three different fungal pathogens Leveillua taurica, Oidium neolycopersici, and O. lycopersici. Notable vegetable high tunnel host crops include tomato, pepper, and eggplant.

Close-up of Botrytis cinerea conidiophore with conidia.

Close-up of Botrytis cinerea conidiophore with conidia.

P. Bachi, University of Kentucky

Close-up of Passalora fulva conidiophore with conidia

Close-up of Passalora fulva conidiophore with conidia

P. Bachi, University of Kentucky

Powdery mildew spores in chains on leaf tissue

Powdery mildew conidia in chains on leaf tissue under a dissecting microscope

Inga Meadows

Symptoms and Signs

Gray Mold can occur on all aboveground plant tissue. The most characteristic sign of gray mold is the profuse fuzzy, gray-brown growth of conidiophores (spore-bearing structures) from necrotic plant tissue. Lesions on leaflets will expand to cover the whole leaflet, to the petiole, and finally the stem. Stem lesions may girdle the stem and cause wilting above the lesion. Stem lesions are the primary cause of plant death in high tunnels. On fruit, B, cinerea causes typical soft rot symptoms and decayed areas are whitish in color, which may contain sporophores. B. cinerea can also produce ghost spots characterized as necrotic flecks surrounded by whitish halos that are 3 to 8 mm in diameter on the fruit. Ghost spots result from aborted infections by spores of B. cinerea and typically occur on mature-green fruit.

Gray mold lesions can be confused with lesions caused by other fungi, damage caused by high salt content in the soil, or wind damage. Gray mold can be distinguished from other disorders by the presence of conidiophores on the surface of diseased, necrotic areas, although spores may not always be present. Spore clouds can be shaken from the conidiophores after prolonged periods. Spores (conidia) are colorless, single celled, ovoid, and are 7.3 x 8.0 to 9.7 x 11.1 µm in size.

Leaf mold primarily occurs on the foliage, but may spread in some cases to the petioles, stems, blossoms, and fruit. Older leaves are affected first and then the younger leaves. Leaf mold initially appears as pale-green or yellowish spots on the upper leaf surfaces that later turn yellow. On the underside of leaves, an olive-green mold is associated with the spots. The mold is more dense and deeper in color in the center of the spots. Spots can coalesce (grow together) and kill the foliage when infection is severe. Leaves may curl, wither, and drop from plants. In some cases, green and mature fruit may develop a black, leathery rot on the stem end.

Leaf mold may be confused with Botrytis gray mold, late blight, and powdery mildew. Under a dissecting microscope, P. fulva has a buff to brown and velvety appearance. Under a light microscope, spores can be readily observed in diseased tissue. Conidia are pale to dark brown, have zero to three septa, contain a thickened hilum (microscopic indentation), and are 4-10 x 12-47 µm in size. Conidiophores (spore-bearing structures) of P. fulva are ≤ 200 µm in size, unbranched, and restricted at the base with a broader tip.

As noted earlier, powdery mildew of tomato is caused by three pathogens. The most common symptom of powdery mildew caused by L. taurica is the development of small (≤ 1 cm), irregularly shaped, light-green to bright yellow lesions on the upper leaf surfaces and will first appear on older leaves. Lesions may become necrotic, coalesce, and expand resulting in the death of entire leaflets. A powdery sporulation typically occurs on the undersides of leaves but may occur on both sides under conditions of humidity. In contrast, the most common symptom of powdery mildew caused by species of Oidium is the appearance of white, powdery growths of mycelia and conidia on the upper leaf surface. These areas can coalesce and form dense powdery sporulation over much of the leaf surface. Severely infected leaves can become chlorotic and necrotic.

On infected plant tissue, L. taurica will produce long conidiophores that are often branched with conidia that are either cylindrical or pyriform (pear-shaped) in shape. Conidia can be formed singly or in short chains. Cylindrical conidia are 16-23 x 45-65 µm in size and pyriform conidia are 14-24 x 50-71 µm in size. In contrast, O. neolycopersici produce erect, long, unbranched conidiophores with a single conidium. However, under conditions of high humidity, spores can be produced in chains of two to six. Conidia are ellipsoid-ovoid in shape and are 10-20 x 22-46 µm in size. The morphological characteristics of conidiophores and conidia produced by O. lycopersici are similar to those of O. neolycopersici. However, conidia are formed in chains of three to five.

Gray mold leaf lesions caused by Botrytis cinerea on tomato

Gray mold leaf lesions caused by Botrytis cinerea on high tunnel tomatoes

Inga Meadows

Ghost spots caused by botrytis cinerea on tomato

Ghost spots caused by Botrytis cinerea on tomato

NCSU Plant Disease and Insect Clinic

Gray mold fruit lesions with sporophores of Botrytis cinerea

Gray mold fruit lesions with conidiophores of Botrytis cinerea on tomato

Inga Meadows

Underside of a tomato leaf with leaf mold spots

Underside of a tomato leaf with leaf mold spots caused by Passalora fulva

NCSU Plant Disease and Insect Clinic

Upper side of a tomato leaf with distinct yellow leaf mold spots

Upper side of a tomato leaf with distinct yellow leaf mold spots caused by Passalora fulva

NCSU Plant Disease and Insect Clinic

Powdery mildew symptoms caused by Oidium spp.

Powdery mildew symptoms caused by Oidium spp.

Inga Meadows

Disease Cycle and Epidemiology

There are several sources of inoculum for gray mold. B. cinerea has a wide host range and disease can be present on perennial plants in any geographical location. Spores are easily windborne. Sporulation can occur under wet conditions with high humidity (>80%) and moderate temperatures (65 to 75°F). Gray mold begins in relatively cool weather and does not require prolonged periods of high humidity. B. cinerea can survive as a saprophyte on organic matter in the soil. The fungus can also survive from season to season as sclerotia, which form on woody tissues of tomato plants. The fungus will grow from sclerotia or organic matter in the soil and can infect leaves lying on the ground. B. cinerea can infect plant tissue directly or through some type of wound. Thus, use good pruning techniques to avoid excessive damage to plants.

P. fulva, the leaf mold pathogen, needs excess water on foliage or high humidity (>85%) to grow and can cause disease at any temperature ranging from 40 to 94°F. However, disease development is optimal at 72-75°F. P. fulva can survive for at least 1 year as a saprophyte on crop residue and in the soil as conidia or sclerotia. Conidia are spread by wind and rain. The fungus can also be spread from plant to plant via tools and clothing.

The powdery mildew pathogens have a broad host range and can infect several crop and weed species. Weeds and other solanaceous crops can serve as inoculum sources. L. taurica, O. neolycopersici, and O. lycopersici require high relative humidity (75-85%) and can cause disease at any temperature ranging from 50 to 98°F with 70°F being optimal for spore germination. Infection can occur with little or no free moisture under high humidity. Spores are readily windborne and secondary infections also occur under conditions of high humidity.

Gray mold (Botrytis blight) caused by B. cinerea on geranium

Gray mold (Botrytis blight) caused by Botrytis cinerea on geranium

S. Sharpe NCSU

General Disease Management

Losses from Botrytis gray mold, leaf mold, and powdery mildew can be reduced by adopting the following management strategies:

  • Wider plant spacing and improved ventilation can reduce disease incidence. Raise the sides of high tunnels to improve air movement. Use fans to replace the air multiple times per day especially during rainy or cloudy periods. Air movement inhibits Botrytis spores from germinating.
  • Remove lower leaves as the plant ages to improve air movement. Suckering below the first flower cluster on tomato plants will promote air circulation, but be sure the particular variety will respond as desired to suckering.
  • Manage temperatures to reduce condensation. Fans or tubing can be used to distribute heated air throughout the structure. Heating under the bench can also reduce condensation on the plants and reduce disease.
  • Trays, benches, tools, stakes, twine, wire, high tunnel structures, and any other tools should be washed and sanitized between crops.
  • Workers should wash hands often—at least at the end of each row—to minimize spread of pathogens.
  • Eliminate weeds from the high tunnel. Weeds can harbor diseases that can affect vegetables.
  • Scout for disease routinely.
  • Remove infected seedlings and diseased tissue.
  • Remove and destroy crop residue.
  • Place cull piles away from the high tunnel.
  • Avoid combining ornamental plants with vegetable production in high tunnels. Some ornamentals can harbor diseases that also affect vegetables.
  • Avoid overhead irrigation. Use drip tape or water at the base to reduce moisture on leaves and splash dispersal of pathogens.
  • Avoid damaging plants, especially for the prevention of Botrytis gray mold.
  • Avoid handling wet plants.
  • Crop rotation can reduce disease incidence but can be difficult to achieve in high tunnels. Mobile high tunnels can be moved from one location to another but must be securely anchored to withstand high winds. For immovable structures, crops should be rotated within the high tunnel between plant families.
  • Use resistant varieties when possible. For more information on specific varieties please refer to the 2019 Southeastern US Vegetable Crop Handbook and Cornell University’s Tomato Variety Tables for more information.
    • There are no resistant varieties to Botrytis gray mold.
    • Varieties resistant to leaf mold such as ‘Trust,’ ‘Geronimo,’ and ‘Starbuck’ should be used as part of an IPM program. However, there are currently 12 races of P. fulva and each variety only protects against only one to a few races. Currently, it is unknown which P. fulva races are present in North Carolina.
    • There are some tomato varieties such as ‘Geronimo’ and ‘Toreo’ that are tolerant to powdery mildew that can be used as a part of IPM program.

Disease Control for Conventional Growers

There are several products that are labeled for use on greenhouse and high tunnel tomatoes for managing Botrytis gray mold, leaf mold, and powdery mildew (Table 1). This is not an exhaustive list of all available products, for the latest fungicide recommendations see the Southeastern US Vegetable Crop Handbook. Additional products labeled for field use can be used in high tunnel production when sides are open. All products provide the best control if applied preventatively. According to the Fungicide Resistance Action Committee (FRAC), B. cinerea has been categorized as a high-risk pathogen due to the development of fungicide resistance and resistance management is required.


Table 1. Fungicides labeled for use on tomato to manage Botrytis gray mold, leaf mold, and powdery mildew in greenhouse and high tunnels.

Active Ingredient

Example Product

PHI (days)1

FRAC Group2

Disease

CyprodinilR + FludioxonilR

Switch 62.5 WG

0

9 +12

Botrytis gray mold

Powdery mildew

Fenhexamid3

Decree 50 WDG

0

17

Botrytis gray mold

Fludioxonil3,R

Spirato GHN

0

12

Botrytis gray mold

Powdery Mildew

Fluopyram + pyrimethanilR

Luna Tranquility

1

7 + 9

Botrytis gray mold

Powdery mildew

Hydrogen dioxide + peroxyacetic acid4

ZeroTol 2.0

0

NG

Botrytis gray mold

Leaf mold

Powdery mildew

Mancozeb5

Various

5

M3

Botrytis gray mold

Leaf mold

Penthiopyrad3

Fontelis 1.67 SC

0

7

Botrytis gray mold

Leaf mold

Powdery mildew

Potassium bicarbonate4

Carb-O-Nator

MilStop

0

NC

Botrytis gray mold

Powdery mildew

PyraclostrobinR + boscalid3,R

Pageant Intrinsic

0

11 + 7

Botrytis gray mold

PyrimethanilR

Scala SC

1

9

Botrytis gray mold

Sulfur4

Various

See label

M02

Powdery mildew

Trifumizole

Terraguard SC

1

3

Powdery mildew

1 PHI= preharvest interval
2 FRAC code (fungicide group)
3 Do not make more than two consecutive applications before alternating with fungicides that have a different mode of action. For Pageant Intrinsic, do not make more than one application before alternating with fungicides that have a different mode of action.
4 OMRI listed products available.
5 Avoid repeated applications of ethylene-bis-dithiocarbamates (i.e. mancozeb) fungicides as they may increase the severity of gray mold in high tunnel production.
R Resistance to this fungicide group is known to occur in Botrytis cinerea.


Useful Resources

Acknowledgments

This disease factsheet was prepared by the Meadows Plant Pathology Lab.

Funding for creating this fact sheet comes from the United States Department of Agriculture (USDA)-National Institute of Food and Agriculture (NIFA) (2017-70006-27141).

Authors

Postdoctoral Research Scholar
Entomology & Plant Pathology
Extension Associate, Vegetable and Herbaceous Ornamental Pathology
Entomology & Plant Pathology

Publication date: May 8, 2019

North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation.

Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by NC State University or N.C. A&T State University nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your local N.C. Cooperative Extension county center.

North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation.