I have a couple of questions about the attached photos of my cucumber and cantaloupe plants:
1. Can you identify the disease?
2. The plants are still growing and green on the ends farthest from the damage. Would it be best to pull them now, or can I let them continue to grow in hopes of a harvest?
3. Whatever the disease, I've had problems with it for a few years. Is it soil-borne? Do I need to stop growing cucurbits for a while?
Thanks for your time!
Monroe County Michigan fruits and vegetables
Dear cucumber grower,
Thank you for sending these great pictures. It appears that your cucumbers may have a fungal disease called Downy mildew, which is caused by a fungal-like pathogen that affects the leaves of cucurbits, particularly cucumbers. Downy mildew has been documented this year in your county.
Management recommendations are to increase spacing between plants, remove infected material and limit overhead irrigation. It cannot be cured with a fungicide, but can be prevented if applied before the infection shows up.
The symptoms are angular lesions on the leaves. The underside of the leaves have tiny gray or brown sporangia that look very similar to dirt. Downy mildew and powdery mildew can look very much alike at first glance, but they are actually quite different. Both of these forms of mildew typically only affect the leaves, however you can identify downy mildew by the fungal layer underneath the leaf – this forms in moist weather and is normally paired with leaf spots on top of the leaf. Powdery mildew causes white, powdery, fungal growth on the leaf.
Powdery: Spots are circular, not limited to leaf veins. There is yellowing, but only after powdery mildew fungus has been present for some time; fungus grows on leaf surface spores produced in a single chain on a single stalk and grows on either leaf surface.
Downy mildew spots tend to be angular and limited by leaf veins. Yellowing develops with downy mildew infections sometimes before fungus is evident. Fungus grows inside the leaf and produces spores on the surface. Spores are produced singly on the end of branched stalks. The disease tends to be more visible on the underside of leaf surface as fungus sporulates through stomata in the leaf.
These websites have a lot of information on the disease and how to control it. It is suggested that you start early in the year, before the disease has a chance to infect your plants. Weather has a lot to do with the spread of the disease, which is prevalent in wet weather. So, remember not to water them overhead, keeping the leaves as dry as possible, prune out the infected leaves and apply a fungicide early in the growing season. As to your other questions, Downy mildew is not in the soil, but is spread through wind blown spores from infected plants. It does not over-winter in soil. So continue your planting, but be vigilant, using preventive measures.
I hope this was helpful. Feel free to contact us again if you have further questions.
Thank you so much for your thorough and speedy answer to my question Ruth! What a wonderful service.
Do you think I should immediately remove the vines that are currently growing? They're still producing blossoms, and some of the leaves are unaffected.
Dear cucumber grower,
I suppose it would depend on how much of the vines are infected. If you can remove and dispose of the infected leaves and still have a plant that is producing, I would do that. The disease is on the infected leaves and you need to get rid of them. The spores spread through water, so if you can remove the infected parts of the plants before it rains again, it would definitely be advisable. Also, remember to water them only on the soil, not their leaves. Good luck with this. We are always happy to help. Thank you for the compliment.
Immense thanks Ruth!