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Gummy stem blight and black rot of cucurbit crops   

This disease has been observed sporadically and uncommonly on Long Island, likely reflecting the pathogen, a fungus, not being established and surviving in the area rather than conditions not being favorable. Most important is high humidity and plant tissue (in particular leaves) staying wet for at least an hour for infection, longer for lesion development. Frequent rain and overhead irrigation done when foliage won’t dry promptly (such as when irrigation ends after sunset) promote disease development. Infection occurs when warm (64-82 C), with 75-77 being optimal. This disease is more common in humid southern U.S. states, as well as in subtropical and tropical areas of the world.

The pathogen can be seed-borne.  Its spores are moved by splashing water and wind.  It survives in crop debris. Three species of Stagonosporopsis cause this disease. The pathogen’s previous name was Didymella bryoniae.

All cucurbit crops are susceptible.  Leaves including cotyledons of all melons are especially susceptible while those of cucumber and some squashes are initially resistant and become susceptible as they age.  Butternut squash and summer squashes are least susceptible.  Melons are more susceptible to stem cankers than other cucurbits while stems of butternut and related squashes (Cucurbita moschata) are resistant.  Fruit rot is most common in pumpkin and winter squashes.

Gummy stem blight is the disease name used when symptoms develop on stems and leaves. Leaf spots initially are round or triangular when beginning at the leaf margin.  They can rapidly enlarge becoming irregular.  Color is tan to brown.  When seedling stems are infected the tissue often appears water-soaked and can lead to girdling which kills the plant. Petiole and stem tissue affected on older plants is water-soaked, tan, and develops a characteristic reddish brown gummy exudate.  Tiny black specks, which are the pathogen’s fruiting bodies, develop on infected plant tissue including fruit.  They are visible in the second photograph below which is a close up of leaf tissue.

Black rot is the disease name used when symptoms develop on fruit. On pumpkin and winter squash symptoms are brown to pinkish and water-soaked at first, then become black, except affected tissue on butternut squash develops white to orange-brown concentric rings.  Tan to brown discoloration occurs on cantaloupe fruit and the netting does not develop completely and is disintegrated.  Black decay can develop on cucumber before harvest but more commonly during transit or storage.

Management: Purchase seed that has been tested for the pathogen and fungicide treated.  Resistant varieties are not commercially available yet.  Disease development and pathogen spread is a concern when growing transplants, especially melons due to susceptibility of seedling tissue, favorability of conditions in greenhouses, and increased opportunity for pathogen spread due to close proximity of seedlings. When growing transplants, provide separation when possible, such as alternating rows with trays of cucurbits with trays of other vegetables to minimize opportunity for spread.  Keep humidity in greenhouse below 80% with ventilation.  Water in the morning when leaves are dry and conditions facilitate fast drying.  This is important when growing transplants and also in the field where preferred drip irrigation is not feasible.  Avoid planting multiple cucurbit crops together.  Rotate out of fields where cucurbit crops were grown previously for at least 2 years (4 years recommended after a confirmed occurrence).  Check plants routinely for symptoms.  Apply fungicides on a preventive schedule.  Bravo is a good protectant fungicide.  Select targeted fungicides when the disease has been confirmed present in a crop.  The pathogen has demonstrated ability to develop resistance to fungicides, including those in FRAC code 1 and 11, plus some in FRAC code 3 and 7 (Endura and Pristine).  Therefore it is important to have current information about resistance occurrence when selecting fungicides.  Avoid injuries to fruit which enable the pathogen to infect.  Before storing winter squash, harden the rind by curing at about 77 F for 1 – 2 weeks.  Optimum storage conditions are 55 (52-61) F and 60 (55-75) % RH.  Promptly incorporate plant debris after harvest.

gummy stem blight on melon leaf and stem

gummy stem blight on melon leaf

black rot on pumpkin

 

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