I am intrigued by altered growth in plants. Think distortion, witches’ brooms, mosaics, mottling and other unusual stuff. Some have a biotic cause (pathogen, pest) but abiotic factors (particularly herbicide damage) also occur. My interest in such oddities began with a visit to Honduras. It was the first day of a new project and Jill Lenné and I were puzzled and intrigued by weird patterns of growth on Gliricidia sepium, a woody legume and key agroforestry species. And not just the odd afflicted tree, but lots of them. I’ve included an example of G. sepium in this month’s quiz.
I’ve included a diverse range of causes associated with unusual growth. These include fungi, bacteria, phytoplasmas, herbicide damage and even insects – this is the age of inclusivity. Compare healthy with altered growth. Some photos show detached symptoms next to healthy growth.
I find the best approach is to eliminate improbable or unlikely causes first. Reduce the possibilities and you get closer to the more likely cause. Beware the tendency to diagnose unusual symptoms as ‘nutrient deficiency’ or ‘virus’. Fungi are not the only plant pathogens and viruses are not the only cause of altered growth.
These are not easy symptoms to diagnose so thanks for having a go. In future, keep your eye out for unusual growths as you garden or go walking. There are more oddities around than you might realise.
Dr Eric Boa
#1. Bramble (if you’re Scottish) or blackberry shoot (Rubus sp,), an unwelcome invader from my neighbour’s garden that I have tried to get rid of. Look carefully at the small clusters of leaves and compare with the normal leaves on the far left. Is this a biotic disease or could it be something else?
The correct answer is:
Herbicide damage. Look out for bunches of string-like leaves, as shown here. Glyphosate doesn’t always work and although the blackberry canes have been damaged, they continue to grow. Damn.
Comments on the other options:
Phytoplasmas can stimulate growth and have been recorded on Rubus spp. in the UK, though normally associated with stunting. Lack of nutrients will reduce growth, not stimulate it. Insect (or rather arthropod – mites usually) feeding can produce unusual growth, but not here.
#2. Outgrowth on wild rose. What is the cause?
The correct answer is:
Insect damage: Diplolepis rosae, a wasp. It appears to have several common names: rose Bedegaur gall; Robin’s pincushion
Comments on the other options:
Crown gall occurs on stems, not on leaves, and this is a fleshy rather than woody growth. Herbicides can interrupt normal growth, but the symptoms would occur over a wider area of the plant (see q1).
#3. Maize cob with black swellings. What is the cause?
The correct answer is:
Fungus. This is maize smut (Ustilago maydis). Smuts infect floral parts and produce black powdery masses, and, in this case, considerable swelling. Smutty cobs are edible and much liked in Mexico. I’ve eaten them.
Comments on the other options:
The powdery masses are a bit of a fungus give away. Excess nitrogen wouldn’t result in swelling. Early frost would kill tissue, not cause new growth. There are phytoplasmas on maize which interfere with general growth e.g. maize reddening and corn stunt.
#4. This is a common ornamental (Codiaeum) used in hedges and other barriers. This is in Sri Lanka. My good friend Ari is smiling because I’ve just told him what this is. He thought this was normal growth, but it’s not. What made him smile?
The correct answer is:
Phytoplasma. I was first alerted to this ‘infection’ by my colleague Yaima Arocha. She spotted the strappy, thin leaves and contrasted them with the broader leaves (look bottom left) in Uganda. The plants tested positive for phytoplasma. Such symptoms are widespread in the tropics, though whether you consider this plant diseased is another matter.
Comments on the other options:
Yes, it could be a virus, but I’d expect to see less vigorous growth and perhaps death of leaves. You could argue that this is a varietal characteristic: this is what people are used to seeing and the plant is growing vigorously. Poinsettias are ‘infected’ by a phytoplasma and sold on their compact shape. Definitely not insect feeding. Ari was smiling because I saw an infected plant and he saw normal growth.
#5. Maize in South Africa, photographed by my colleague Wilmarie Kriel. What’s the cause of this grassy-like bundle?
The correct answer is:
Downy mildew or crazy top of maize, caused by Sclerophthora macrospora.
Comments on the other options:
There are many viruses on maize with common leaf symptoms such as streaks, mottles and so on. Not grassy clusters. There are two phytoplasma diseases I know of on maize, one causing reddening and the other stunting. ‘Lack of nutrients’ is a diagnosis of last resort, mainly, I suspect, because of a lack of awareness of less well known causes e.g. downy mildew.
#6. Coconut palm plantation in Cote d’Ivoire. Fronds were collapsing all around me, one of the most dramatic examples of a plant health problem I have seen. This problem also found in East Africa, the Caribbean and Central America. Coconuts are attacked by many pests and diseases. Which one? Or is this an abiotic problem?
The correct answer is:
Phytoplasma. Lethal yellowing of coconut palms, known locally as St Paul’s wilt, is a devastating disease, with variants found around the world. The sudden wilting of the canopy is linked to the ultimate collapse and failure of conducting vessels (phloem).
Comments on the other options:
Rhinoceros beetle is also damaging but crowns tend to become ragged and lack this sudden collapse of fronds. Waterlogging would affect more palm trees at the same time. Fungal root rot is a ‘good’ guess, and Phytophthora is widespread. Difficult to distinguish without testing of samples.
#7. Detached branches of Gliricidia sepium, a widespread agroforestry species in tropical regions. Compare the unhealthy, yellowed foliage on the left with the normal growth next to it. Both occur on the same tree. The symptoms are distributed unequally throughout the crown. Some of the branches have started to die.
The correct answer is:
Phytoplasma. Look at the clumped growth, reduced internode length and reduced leaflet size. The difference between unhealthy and normal growth is stark and dramatic – a good starting point for thinking ‘phytoplasma’
Comments on the other options:
Yes, it’s similar to herbicide damage – but drift is unlikely to reach the top parts of the crown. Virus infection is also a possibility; distinguishing little leaves from reduced growth is not easy and requires close examination (even then I’m easily confused).
#8. What is the cause of these symptoms on Peach trees?
The correct answer is:
Yes, it’s another phytoplasma, this time peach yellows. Widespread in Europe with variable yet common symptoms: distorted leaf development, distinct yellowing and dieback
Comments on the other options:
Birds eat leaves; they don’t alter growth. The symptoms do look virusy and this is a reasonable guess given that the photograph is taken from a distance (in Cyprus). Prolonged lack of water would result in a general decline and loss of leaves all over.
#9. Leaf curl in field-grown tomatoes. What is happening?
The correct answer is:
Wind damage. There are probably other factors which contribute to a temporary response to localised water loss. I’ve often seen it early in the morning, disappearing later in the day.
Comments on the other options:
Mite feeding causes leaf browning and the plant becomes ragged. Both viruses affect the lamina of the leaf, with changes in colour and reduced growth. When leaf curling affects an otherwise an unmarked leaf then start to suspect a non-pest cause. Check if leaves have been affected at the same time (as seen here), suggesting a sudden change and upset to growth, such as frost or herbicide damage. Diseases develop progressively.
#10. Why this dramatic difference in growth, compared to the surrounding plants? Seen in South Africa
The correct answer is:
Uneven application of cow manure. The farmer had been encouraged to add the manure by local extension officers, without them realising that supply was limited.
Comments on the other options:
The plants in this small patch are growing normally. There are no obvious symptoms – marks on the leaf, changes in colour, dying leaves and so on. There’s no sign of irrigation pipes. Different seed varieties would show less marked growth differences and mature plants would not occur in a distinct patch. And no, it’s not a phytoplasma.
Results
We hope you enjoyed the quiz and learned something!
See how your score matches up…
1-3: You need to do a few more of these quizzes to become a field diagnostics expert.
3-5: You have made a start in your field diagnostics but there is some way to go…
5-7: You are getting more right than wrong so keep on learning!
7-9: Impressive you are getting pretty experienced in your field diagnostics!
10: Well done! But make sure you do the quiz again next month.
We hope you enjoyed the quiz and learned something!
See how your score matches up…
1-3: You need to do a few more of these quizzes to become a field diagnostics expert.
3-5: You have made a start in your field diagnostics but there is some way to go…
5-7: You are getting more right than wrong so keep on learning!
7-9: Impressive you are getting pretty experienced in your field diagnostics!
10: Well done! But make sure you do the quiz again next month.
