Answers to Your Midsummer Gardening Questions

By Steve Reiners, Professor and Chair, Horticulture Section, Cornell University, Cornell AgriTech. This is part of a series of columns that he wrote about vegetable gardening during the pandemic that ran Spring-Summer 2020. Read more articles from 2020 | Read later articles in this series.

July 17, 2020

Always nice to hear from people with questions.  Here are a few I received this week.

My husband planted a new garden this year.  The zucchini, pumpkins and cucumbers look great.  The tomatoes started ok but are wilting.  My neighbor says it’s because of the nearby walnut trees.  What is he talking about?

Black walnuts are a common tree here in New York and they have a very unique ability.  Like other members of the walnut family (butternuts, hickories), they produce a chemical called juglone.  Juglone is produced in fruit, leaves, branches and the roots.  And some can be excreted by the roots.

Unfortunately, juglone can injure and even kill some susceptible plants, and tomatoes are one of the most sensitive.  The plants may first start to yellow and then eventually succumb to “walnut wilt”.  I think this is happening to your tomatoes.

The greatest amount of juglone occurs within the drip line of the trees (the area between the trunk and the end of the branches), but it can extend as far as 50 feet from the trunk.  This may include your entire yard so what can you do?

Grow tolerant veggies like squash, cucumbers, beans, carrots, onions, garlic, leeks, beets, melons and corn.  In addition to tomatoes, sensitive crops include cabbage peas, eggplant, potato and peppers.

 If you want to grow tomatoes and other sensitive crops, grow them in containers that keep the roots away from the juglone.

My tomato plants look great, but I noticed some of the green tomatoes on the vine are showing brown, dry spots on the bottom of the fruit.  I think it’s late blight but not sure what to do.  Help!

First, let me assure you it is definitely not late blight.  Late blight is a severe disease of tomatoes and potatoes (and the cause of the Irish Potato Famine) and we can get it some years in New York.  But because it is so severe University extension specialists are constantly monitoring for the disease.

The last report I saw was that it is still thousands of miles away.  Plus, it thrives in cool, wet weather which is just the opposite of our summer this year.

What you have is blossom-end rot (BER).    The large, leathery brown spot on the bottom of the fruit is caused by a calcium deficiency in the soil.  The problem is usually not due to low soil calcium, but the soil is too dry.

Calcium is picked up by roots with the water and flows with the water through the plants.  Dry soils mean less calcium in the plants and the resulting BER.  This hot, dry summer has provided perfect conditions for BER.

The fruit are fine to eat as you can just cut off the browned end.  Now’s a good time to check the still green tomatoes on your plants and cull the ones with BER.  Most of the other fruit will likely be fine.  Make sure you keep the plants well-watered and put a mulch around the plants to reduce evaporation from the soil.

If I take off all but a few of the small pumpkins on my plant, will the remaining ones be larger?

In my experience, typical Jack-o-lantern type pumpkins, in the range of 12 to 25 pounds, will only produce 2 to 3 useable fruit.  There will be others, but they usually stay small and green but will use some of the plant’s resources.  So yes, removing any fruit after you have 2 or 3  healthy ones can give you a little larger fruit.

Now’s a good time to scratch a name in your pumpkin too.  Any sharp tipped instrument will work, just scratch about a ¼ inch deep.  The fruit will callus over where you scratched and as the fruit expands, so will the lettering.  You can try this with zucchini too! It’s a fun thing to do with kids.