Why are my zucchini fruit turning yellow?

San Francisco Bay Ar, CA(Zone 9b)

This is a new one for me. I've had plants turn yellow, but never the fruit. A photo of this week's zucchini is attached. I'll include a photo of last week's zukes in the next post.

Any idea why this is happening? I'm thinking it's a nutrient imbalance, but since the leaves are green and growing well, I don't think it is a lack of nitrogen. Too much nitrogen maybe? Iron deficiency? Calcium? Just need more food?

A little background on my garden:
We have a 4x16 raised bed at a new community garden. Bed assignments were given out on Memorial Day, so everyone got a late start to this year's planting. Normally I grow a few sets of green manures when starting a new garden, but due to the late start we just amended the soil and planted. Our native top soil is adobe clay. We've amended the bed with 28 cu ft of compost in addition to alfafa, rock phosphate, oyster shells and kelp meal. Our newly planted seedlings turned yellow in a week, so we've been feeding with a commercial organic fertilizer and I've been spraying with a liquid kelp solution.

We follow a combination of square foot gardening/biointensive techniques. The plants are all growing well with few problems. We had some powdery mildew show up during the last heat wave. I've been spraying with a milk and water solution that is working well. The cucumber beetle damage has been minor so far.

In the past three weeks, I've harvested 3 pounds of genovese basil (which became a quart of pesto sauce, yum!), six "normal" romanesco zucchini, a large salad bowl of golden purslane, and a large bunch of parsley, dill, french tarragon and collard greens. The pole beans have set, the potatoes are blooming, one buttercup is 6 inches in diameter and dark green already. Some of the younger ones further up the vine are starting to look a tad yellow this week like the zukes. Help!

Thumbnail by garden_mermaid
San Francisco Bay Ar, CA(Zone 9b)

Here is a photo of last week's zucchini harvest. They look fine and tasted delicious.

Thumbnail by garden_mermaid
San Francisco Bay Ar, CA(Zone 9b)

Here is a photo of the squash section of the garden bed. The plants seem otherwise healthy.

Thumbnail by garden_mermaid
San Francisco Bay Ar, CA(Zone 9b)

The person at our local organic garden supply suggested that the fruit might be turning yellow because they got cross pollinated with a yellow type of squash. I'm not growing any yellow squash, but I'll check the neighbors garden beds to see if there are any nearby.

Has anyone had this type of cross pollination effect on their summer squash?

Augusta, GA(Zone 8a)

Only if your seeds were cross pollinated. Cross pollination does not affect the current crop. If you are not saving seeds you can mix or match all types of squash in the same bed.

San Francisco Bay Ar, CA(Zone 9b)

Thanks Farmer Dill. It sounded odd to me that squash cross pollination would affect the current plants fruit. Any idea why the zucchini fruit are turning yellow? Have you seen this effect before?

Augusta, GA(Zone 8a)

Not really, There is a zuchinni yellow mosaic virus. Never ecncountered it myself, but supposedly affects the entire plant. http://image.fs.uidaho.edu/vide/descr909.htm http://www.apsnet.org/online/feature/pumpkin/zuccyell.html Does not seem to fit your description. Assuming that other plants grow well in garden, I would suspect a virus. http://www.hort.uconn.edu/IPM/veg/htms/squash.htm California has stress organisms that we in the east are not familiar with.

San Francisco Bay Ar, CA(Zone 9b)

Farmerdill - thanks for the information and insight on the plant stress issue.
The two yellow fruit grew during our exceptional heat wave last week (record triple digits than came on suddenly). It seems reasonable that this may have been due to stress from that period. I will keep an eye on the plant and see if any new fruit are also yellowing.

Sunnyvale, CA(Zone 9b)

Hi Mermaid!
You are a neighbor (I live in Sunnyvale too!)

It sure was hot for a couple of weeks, wasn't it. It is again very nice and I hope this keeps up!

My squash plants are doing fine, but this is my second planting this season, so I didn't have any fruit during the heatwave myself. I have gotten two fruits since then, and they looked fine, but they were of yellow variety anyway, so I didn't see anything unusual.


San Francisco Bay Ar, CA(Zone 9b)

Thanks tmm. There are two new squash forming on the same plant and they seem to be the normal green colour again. So the yellowing appears to be a response to the heat stress.

I found this interesting article on growing in hot conditions:

Winchester, VA(Zone 6a)

My plants turned yellow! And 3 died. What's up with that?

San Francisco Bay Ar, CA(Zone 9b)

pegdog - need a little more information to beging to answer you question. There are many things that can cause a plant to turn yellow.

I posted the original question because the plant was/is healthy, just the fruit was turning yellow.

Did your plants turn yellow during a heat wave? Did the entire plant turn yellow or was there a pattern to the yellowing, eg, did the lower leaves turn yellow first? Didi it turn yellow evenly, or with blotches or veining? Were your plants zucchini or squash?

hayward, CA

hi garden mermaid,

I was happy to hear someone else is having yellowing fruit,

I to had healthy plants till the heat wave then my zuccini was good till the end it turned yellow and was vey soft , now im getting nice zuccini again but some of my leafs have a powdery film on them.. any suggestions?


Winchester, VA(Zone 6a)

Powery mildew: use the "milk" concoction. I will look for recipe, but it is posted many times on this site.

Zucchinis: the leaves turned yellow, the zukes stopped growing and then ... the entire plant wilted and "croaked." :(

Shenandoah Valley, VA(Zone 6b)

pegdog, that sounds like the vine borer's work.

Winchester, VA(Zone 6a)

EWWWW! what does that look like and how do i kill it?

Shenandoah Valley, VA(Zone 6b)

Ha ha ha ha ha ha ... kill it, that's a good one. Sorry. Do I sound bitter? :)

As an adult, it is a black and red moth with clear wings that flies during the day. Scroll down a bit in this forum to "Squash Vine Borer...eggs" thread for pics of everything. I think critterologist's link has good photos on it.

My best defense is to keep sticking zucchini seeds in the ground every two weeks or so, and covering them with superlight row cover until they flower. Eventually the borer will get them, but by then another seedling should be ready to bear. sigh...

Winchester, VA(Zone 6a)

OMIG*D. I can't believe it. So sad news. I am the only "gardener" in my neighborhood that can't make zucchini bread! NO ZUKES!

East Barre, VT(Zone 4a)

lizrainey, the milk spray for powerdy mildew doesn't need to be complicated. I just use one part milk to 9 parts water, and spray the heck out of the plant. Make sure you hit the top and bottom of all leaves, and make sure the splash doesn't hit any other plants. The virus can travel from backsplash. If any part of any of your tools come in contact with the infected plant, make sure you sterilize them before using them on healthy plants. I learned that the hard way last year!

hayward, CA

hi leisurlee,

thank you so much for the recipe, i'll be out there this evening spraying,
fortunate i had them in a large raised bed by themselves this year, nothing near them
to get infected . thanks again liz

San Francisco Bay Ar, CA(Zone 9b)

LIz, please keep us posted on your zukes after you use the milk spray.

Zeppy, I've been researching the squash vine borer since you are having such a time with them. You mentioned that your plants growing near the clover were less affected. Is that still the case? I was looking for any information on whether the undersown clover or other crop can help with this pest. It appears that the vine borer's range is east of the Rockies, so we have a geographical advantage our west. If you paint Neem on the stems does it help keep them out?

hayward, CA

hi garden mermaid,

well i think the spray was a success, i worked on spraying them down good got rid of all the dead leaves and they have sprung back pretty good lots of new zuccini's,
plants are looking healthy again, leisurlee thanks again for the milk spray idea

happy gardening liz

Middletown, CA

I had major problems with heat stress most of the summer. We got hit with our first heat wave early in June. Not only did we have heat, but we had heat waves alternating with cool spells 115 degrees one day, 75 the next. Throw in a few 45 degree nights. Kept me pretty in my garden. I had all sorts of yellowing of plants, wilting, dropped blossoms etc.

I ended up covering my entire garden with 30% shade cloth and 50 and 65% shade cloth for zucchinis, crooknecks and cucumbers on the west side. This really helped alot. Before I put up the heaver shade cloth, the cucumbers were all dropping blossoms on the side facing west.

Many of my plants didn't really start producing until late July, early August. The weather has settled down some now with temperatures in the 90's.

San Francisco Bay Ar, CA(Zone 9b)

Thanks for reporting your experiences nataraj. I'll probably try your shade cloth technique next summer if we are going to have a repeat of these extreme heat waves.
The squash did drop blossoms and abandon many of the smaller fruits after the first heat wave. Things went back to normal for a while and then that same plant produced yellow tinged zucchini again after the second heat wave.

Rocky Mount, VA(Zone 7a)

Garden I got tired just reading what you did for the prep. No wounder my beds were such a failure this year!

San Francisco Bay Ar, CA(Zone 9b)

LOL Dyson! Starting a new garden can be a lot of work. The good news is, it gets better as you work at it. Fortunately for me, my DH does the double digging.

Good soil is the key. 20/20 hindsight tells me that it would have been cheaper for us in the long run if we'd had a professional soil test done after our initial bed prep. I would have added a lot more rock dust to remineralize and tossed in chicken manure as well. We'll do that when we pull the summer vegie plants and prep for the fall/winter vegies. I'm still working out rotations with my cover crops. Part of me wants to just sow the whole bed with soil builder mix and let it grow until spring. The other part of me wants the cabbages, brussel sprouts, carrots, chicories, parsnips, salsifies etc. I'm sure I'll find a compromise. I can't really complain. We've harvested ~20 lbs of tasty vegies and herbs out of our 50 sq ft of growing area in the last two weeks. There's still a lot more to come in the next few weeks. Next year we are definitely planting the summer crops earlier in the season!

I read through some of your other posts and II wouldn't call your beds a failure! Not at all. In addition to what you've harvested, you've gained insight into what to do differently next year.

Middletown, CA

I've been reading up on cucurbitacae diseases to try to see what went on with my plants. My conclusion is that my cucumbers got the Mosaic virus while they were stressed out from the heat. I don't think my Zucchini's got it.

This could also be a possible explanation for the yellow zucchini's. The mosaic virus is described very differently in each of the many descriptions that I read of it and the only way to confirm it is with lab tests which I'm not about to do. Yellow cucumbers are definitely a symptom of the mosaic virus and I've had some of those from the affected plants in the last 2 weeks.

The only other possible cause I could find for the yellow zuchini's would be that the seeds that you planted (i.e. not pollination of the current plant, but the seed that you grew the plant from) was contaminated by unintentional hybridization and there was a recessive gene from a yellow variety.

Actually there is one hybrid that I saw for sale some where that documented that the plants produced yellow zucchini's initially and then switched to green after the plant matured.

San Francisco Bay Ar, CA(Zone 9b)

Thank you for the research inputs nataraj. I looked at photos and descripions of the mosaic virus when FarmerDill brought it up. I got the impression that a plant infected with the mosaic virus would show some pathology in the leaves as well as the fruit. My plants looked fine otherwise, only the fruit was affected by the yellow colour. There was no deformity or change in skin texture on the fruit, only the odd colour. This dissappeared from subsequent fruit that grew during "normal" summer weather and temps. The yellow fruit appeared again with fruit forming during the second heat wave and again returned to normal colour after the weather settled down. You might be on to something with the unintentional hybridization concept. I have three romanesco varieties planted. Only one plant was affected by the yellow colour. The others were fine.

Did your cucumbers exhibit the mosaic pathology in the leaves as well?

Middletown, CA

Yes, my plants did. They became yellow, but continued to produce new green growth from the tops of the vines. At first I thought it was just heat damage, but after reading more of the symptoms and watching the plants as they got older I concluded it had to be mosaic.

They do say that a plant could be only mildly affected. If the soil and other conditions are quite good, it's possible to see only minimal symptoms, but who knows. Where did you get your seed from?

I am going to really work on getting the brix up on all of my plants for the fall garden and for next spring. I'm sending in soil samples tomorrow for testing.

Oh, and I like the pictures of your garden. I've got extra space for beds. Your welcome to have a few hundred square feet of bed space in my yard if you like!! Be forwarned though that the soil is heavy clay and digging beds is hard work...

San Francisco Bay Ar, CA(Zone 9b)

Very interesting symptoms for mosaic virus. Maybe we did have a mild case in one plant. Our affected zucchini plant continued to produce green leaves from the base, not just the top of the vine. This may be because we grow them vertically rather than letting them mound. I got the zuke seed out of a jar at Common Ground in Palo Alto. They sell seed by the spoonful, although I 've concluded that they do not rotate their stock often, as some of the seed we purchased from the bulk jars failed to germinate. I'll probably order fresh zucchini seed from GrowItalian.com for next year, although we'll probably more space to the trompocino squash (tromba d'albenga) than the zucchini, since they are such vigourus growers and we really like the flavour.

Glad you like the photos of our garden. Thank you for the offer of space. If we could figure out the logistics we'd take you up on it. The origianl soil in the community garden lot was adobe clay as well. My DH soaked it for several days to soften it enough to take the grubhoe to it. It was indeed a lot of work to double dig.

San Francisco Bay Ar, CA(Zone 9b)

nataraj, forgot to ask, which lab do you use for soil testing?

Lewisville, MN(Zone 4a)

On vine borers. If you watch the stems close to the ground, look for a sawdust type thing on the stem. (The leaves may be wilting.) Take a knife & slit the stem lenghtwise by the secretion. You will find the bore there. Dig him out & mound dirt up to & above the cut. The stem will heal & continue growing.

Middletown, CA

Garden_mermaid: Funny that you should mention about seed problems. Just the night before your post I had a phone conversation with a friend of mine that works for common ground, and shared that I one particular supplier of organic seeds seemed to be more variable in seed quality. (I don't get my seeds from common ground, cause I live too far away, but they get theirs from mostly the same sources that I get mine.).

She said that nobody has ever mentioned having any problems with any of their seeds, but my sense is that they are open to feedback on that.

While I don't know this for sure, from my experience with nutrition and health, it is my guess that organic and/or untreated seeds are going to have a lower germination rate than seeds treated with chemicals to conteract the effect of phytates and enzyme inhibitors, nature's way of adding a time delay to the germination of seed, so that it does not germinate until the right time for a plant to begin it's growth cycle.

So very young untreated seeds, may not have a good germination rate. I experienced this this year, when an early planting of zucchini's had a very low germination rate, but later on in the season, I got a very high germination rate from seeds of the same plant.

I generally start my seeds in flats and pick the best seedlings to transplant, so it doesn't bother me that the germination rate may be low, but still I'd rather have untreated seeds. What matters more to me is the quality of the plants that I get from the seeds that do germinate.

I've had very good results with seeds from http://www.bountifulgardens.org , http://www.reneesgarden.com and Peaceful Valleys own brand of seeds which they give away for free depending on the amount of orders you place with them. For herb seeds, I use mostly http://www.horizonherbs.com . I may get some less common heirloom tomatos from tomatofest next year.

Middletown, CA

The link for peaceful valley is http://www.groworganic.com

San Francisco Bay Ar, CA(Zone 9b)

nataraj, I usually buy organic, untreated seed and try to buy heirloom varieties. It looks like we buy from many of the same seed suppliers. I do buy some seeds from the GrowItalian.com site or from Gourmet International and sometimes from Seeds of Change when I am looking for varieties that the other places do not carry. Many of the heirloom packets will list germination rates. I always plant extra to account for low germination rates since I can usually find homes for extra seedlings with other gardeners. We store our extra seed in cool, dry, dark conditions. CG stores their bulk seed in clear glass containers that are exposed to fluctuating store temperatures. Sometimes it is very warm in the store. Don't get me wrong, I like them a lot. I still buy seed from them when I want to try something new. I just don't count on the same level of germination from the bulk jars as from my other suppliers.

Middletown, CA

What has been your experience with the seeds of change seeds? I find that some of their stuff grows very well, but I've also had a few things that produced runt'ish seedlings or seedlings with defomed cotyledons.

I ordered daikon seed and radicchio today from http://www.seedstrust.com , cause seeds of change was out of daikon. I've never ordered from them before. Very few places sell daikon seeds. I'm going to plant them as a covercrop to loosen up the clay. I also like them for eating and juicing, including the greens.

I love trying all different kinds of varieties of things, and it's nice sometimes to order from some of these importers and heirloom specialists. Generally I don't choose hybrids, but I did get a hybrid cucumber seed from Reneesgarden that turned out to be much more disease resistant and heat tolerant than some of my other cucumbers.

The thing with the seed jars is funny, because John is fanatical about how to store seeds correctly. They could at least get some amber jars, or put paper around the outside of the jars. Bountiful gardens sells moisture absorbing packets to put in with the seeds, though I haven't gotten to those yet.

There are just endless things that we could do, but at some point it's just nice to do nothing for a while. The real challenge is to take care of all of the things that we need to in our everyday lives and do nothing simultaneously.

This message was edited Sep 14, 2006 5:56 PM

Middletown, CA

Up until now, I hadn't really done any soil testing. I have decided to work with International Ag labs. http://www.aglabs.com/ and try their http://www.highbrixgardens.com/ program on some of my beds.

I've learned alot about soil quality and plant health in the last year and it really seems attractive to me to work with somebody who can do testing and guide me in bringing up my soil quality and at the same time producing high brix veggies in a short time frame, something that could take years doing it by guess work.

International Aglabs may not be right for everyone, so I suggest that others talk to different labs and choose a testing service that's right for what they want to do. In particular, don't expect that you can take a soil test report from a specialized lab like this into your local garden store and have them help you decide what to do with it. I am choosing them for testing because I am going to engage their other services and have them guide me.

Ag Labs does a less common type of soil testing called the Morgan test, which more accurately estimates nutrient content based on the plants ability to absorb and use them and does not report unusable nutrients like some other testing methods. Their testing is oriented toward balancing minerals (i.e. rock dusts) and microbial life and may make recommendations for soil ammendments not found in ordinary gardening stores. I don't know if this is the case or not. Also, you can't necessarily compare the results of one labs tests with previous years results from a different labs.

Several links that have writeups on different soil testing labs are:


Several other labs that I've heard good things about are:
Timberleaf (listed in above links) - I suspect that may use morgan testing as well, but I didn't check. They do offer microbial analysis, but it is a seperate test from their basic test.

http://www.agrienergy.net - offers morgan testing as well as a combination of the stuff from the high brix folks as well as other methods.

Both agrienergy and INnat'l Ag labs offer brix raising foliar spray and rock dust products.

There is one other lab that does the high brix stuff, but I can't remember it. It's somewhere in the archives of the brixtalk list.

Peaceful Valley Farm Supply http://groworganic.com and Harmony Farm Supply http://www.harmonyfarm.com/ both offer soil testing and have knowlegable people in house who can guide in the selection of off the shelf products to balance soils.

San Francisco Bay Ar, CA(Zone 9b)

nataraj, I'm seeing a karmic pattern here. :D You seem to be traveling the same web information path that I've been on. LOL!
I was looking at the basic soil analysis with recommendations test from International AgLabs and have been having an email chat with one of their soil consultants, who coincidently spent several years in the homeland of some of my ancestors, so I'm about to send him a masala recipe. I would be interested in hearing your experiences with their program recommendations.

BTW, if that was your post on BrixTalk, congratulations on the 22 brix grapes!

Middletown, CA

garden_mermaid, I sent in my soil test last week. I will let you know when I get the results. Yep, those were my 22 brix grapes. Thanks. I've got more work to do on other things. In particular, the stuff growing in wine barrels is more challenging, cause I have to apply fertilizer more regularly and it's not practical to get soil tests on 20 different containers.

On the other hand, the gophers don't touch the wine barrels. I really got cleaned out in the last two weeks. I lost a good size tomato plant. Two large zucchini plants, a crookneck and an okra. So far, I've not done so well with trapping them. I've considered other methods, but would prefer to trap them. They love my deeply double dug beds, but I don't normally irrigate the hard clay outside the beds and it's hard to follow the tunnels. I set some traps baited with some carrots behind them. Twice the gophers somehow got around the two traps, which were back to back and took the bait in the middle. After that they closed off the tunnel and never came back.

San Francisco Bay Ar, CA(Zone 9b)

Have you tried planting sour clover (Melilotus indica) to deter them? Supposed to be a traditional way to keep them away.

Scroll down to the Gopher Stopper clover section.

You might want to google around on the plant to for more information.

I'm sending in my soil sample today. We'll see what comes back. I'm requesting the organic recommendations, even though that will likely take longer to raise the brix. Some of the foliar sprays for getting calcium into the cells of the leaves and fruits are not on the organic materials list for all certififying groups. It will be interesting to see the impact.

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