BSPP ‘Plant Doctor’ Quiz #20 – November 2022

The correct answer is:

Fungus

Not many fungi cause such total tissue collapse as seen here, but this is a Sclerotinia rot. The presence of the hyphae indicates that it is not a bacterial pathogen and the presence of black cannon-ball sclerotia (fungal resting spores) indicate it has to be a fungal pathogen.   Bacteria never produce structures large enough to be seen and Phytophthora (a very unusual pathogen on lettuce) would not produce the sclerotia.

The correct answer is:

Fungus

Any of the suggested answers can cause leafspots, but there are various clues to point to this being a fungal pathogen: the spots are all of a similar size, they are not associated with the edge of the leaf or with damage and they are discreet and not spreading up or down the leaf veins. All of these features indicate (but do not prove) the pathogen is a fungus. This is Cercospora lactucae-sativae (cercospora leaf spot of lettuce).

The correct answer is:

Moth:

The large head capsule, the three pairs of prolegs and the four pairs of little stubby legs all show that, despite its subterranean lifestyle, it is caterpillar and will turn into a moth. This is a Cutworm, they get their name due to their habit of feeding on the base of the plants and often, in the case of a small seedling, they will chew right through it.

The correct answer is:

Mineral deficiency:

This is “tip burn” and is the lettuce equivalent of blossom end rot. In the same way as the tissue at the end of a tomato will break down, because the plant cannot mobilise calcium to the area fast enough, something similar is happening here. The edge of the leaf is growing so quickly it becomes short of calcium and the tissue dies.

The correct answer is:

Downy mildew sporulation

It’s unusual to see a downy mildew sporulating without causing any host reaction, as seen here.  Similarly, it is unusual to see downy mildew sporulation on the upper and lower leaf surface, but that is what is taking place. Bremia lactucae (downy mildew of lettuce).

The correct answer is:

Bacteria

There are some clues to suggest that this is due to a bacterial pathogen.  The water soaking symptoms that arespreading up the leaf veins and the total collapse of the tissue all indicate a bacterial pathogen. Waterlogging may have led to the pathogen infection, but it could not be considered the cause. Compare this with the Sclerotinia infection of question 1. This is Pseudomonas cichorii infection.

The correct answer is:

Physiological condition; no pathogen involved

Thickening, deformity and roughening of leaf texture in lettuce can all occur in response to cold temperatures.  If the tissue is killed it will begin to decay.

The correct answer is:

Fungal infection:

The sporulation of the fungus (Botrytis) is clearly visible on the surface of this heavily infected lettuce. Nematodes rarely attack above ground and bacteria never produce structures large enough to see unaided.

The correct answer is:

Herbicide damage:

It can be difficult to tell what has caused this kind of symptom and it is often necessary to investigate by looking at the spread of symptoms within the field and talking to the farmer. The symptoms look like they could have been caused by a viral infection but there is no yellowing or mosaic which would indicate that is not the case.

The correct answer is:

Virus:

This is a disease called lettuce big vein. The virus is spread by zoospores of a fungus (Olpidium virulentus) released from sporangia that live in the soil and infect lettuce roots. The virus also enters resting spores, allowing it to survive for years in soil, in the absence of a growing host plant.